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MethodHandles.java
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MethodHandles.java
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/*
* Copyright (c) 2008, 2021, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.lang.invoke;
import jdk.internal.access.SharedSecrets;
import jdk.internal.misc.Unsafe;
import jdk.internal.misc.VM;
import jdk.internal.org.objectweb.asm.ClassReader;
import jdk.internal.org.objectweb.asm.Opcodes;
import jdk.internal.org.objectweb.asm.Type;
import jdk.internal.reflect.CallerSensitive;
import jdk.internal.reflect.Reflection;
import jdk.internal.vm.annotation.ForceInline;
import sun.invoke.util.ValueConversions;
import sun.invoke.util.VerifyAccess;
import sun.invoke.util.Wrapper;
import sun.reflect.misc.ReflectUtil;
import sun.security.util.SecurityConstants;
import java.lang.constant.ConstantDescs;
import java.lang.invoke.LambdaForm.BasicType;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Member;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.ReflectPermission;
import java.nio.ByteOrder;
import java.security.ProtectionDomain;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.BitSet;
import java.util.Iterator;
import java.util.List;
import java.util.Objects;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.stream.Stream;
import static java.lang.invoke.LambdaForm.BasicType.V_TYPE;
import static java.lang.invoke.MethodHandleImpl.Intrinsic;
import static java.lang.invoke.MethodHandleNatives.Constants.*;
import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
import static java.lang.invoke.MethodType.methodType;
/**
* This class consists exclusively of static methods that operate on or return
* method handles. They fall into several categories:
* <ul>
* <li>Lookup methods which help create method handles for methods and fields.
* <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
* <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
* </ul>
* A lookup, combinator, or factory method will fail and throw an
* {@code IllegalArgumentException} if the created method handle's type
* would have <a href="MethodHandle.html#maxarity">too many parameters</a>.
*
* @author John Rose, JSR 292 EG
* @since 1.7
*/
public class MethodHandles {
private MethodHandles() { } // do not instantiate
static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
// See IMPL_LOOKUP below.
//// Method handle creation from ordinary methods.
/**
* Returns a {@link Lookup lookup object} with
* full capabilities to emulate all supported bytecode behaviors of the caller.
* These capabilities include {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access} to the caller.
* Factory methods on the lookup object can create
* <a href="MethodHandleInfo.html#directmh">direct method handles</a>
* for any member that the caller has access to via bytecodes,
* including protected and private fields and methods.
* This lookup object is created by the original lookup class
* and has the {@link Lookup#ORIGINAL ORIGINAL} bit set.
* This lookup object is a <em>capability</em> which may be delegated to trusted agents.
* Do not store it in place where untrusted code can access it.
* <p>
* This method is caller sensitive, which means that it may return different
* values to different callers.
* @return a lookup object for the caller of this method, with
* {@linkplain Lookup#ORIGINAL original} and
* {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}.
*/
@CallerSensitive
@ForceInline // to ensure Reflection.getCallerClass optimization
public static Lookup lookup() {
return new Lookup(Reflection.getCallerClass());
}
/**
* This reflected$lookup method is the alternate implementation of
* the lookup method when being invoked by reflection.
*/
@CallerSensitive
private static Lookup reflected$lookup() {
Class<?> caller = Reflection.getCallerClass();
if (caller.getClassLoader() == null) {
throw newIllegalArgumentException("illegal lookupClass: "+caller);
}
return new Lookup(caller);
}
/**
* Returns a {@link Lookup lookup object} which is trusted minimally.
* The lookup has the {@code UNCONDITIONAL} mode.
* It can only be used to create method handles to public members of
* public classes in packages that are exported unconditionally.
* <p>
* As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
* of this lookup object will be {@link java.lang.Object}.
*
* @apiNote The use of Object is conventional, and because the lookup modes are
* limited, there is no special access provided to the internals of Object, its package
* or its module. This public lookup object or other lookup object with
* {@code UNCONDITIONAL} mode assumes readability. Consequently, the lookup class
* is not used to determine the lookup context.
*
* <p style="font-size:smaller;">
* <em>Discussion:</em>
* The lookup class can be changed to any other class {@code C} using an expression of the form
* {@link Lookup#in publicLookup().in(C.class)}.
* A public lookup object is always subject to
* <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
* Also, it cannot access
* <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
* @return a lookup object which is trusted minimally
*
* @revised 9
*/
public static Lookup publicLookup() {
return Lookup.PUBLIC_LOOKUP;
}
/**
* Returns a {@link Lookup lookup} object on a target class to emulate all supported
* bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">private access</a>.
* The returned lookup object can provide access to classes in modules and packages,
* and members of those classes, outside the normal rules of Java access control,
* instead conforming to the more permissive rules for modular <em>deep reflection</em>.
* <p>
* A caller, specified as a {@code Lookup} object, in module {@code M1} is
* allowed to do deep reflection on module {@code M2} and package of the target class
* if and only if all of the following conditions are {@code true}:
* <ul>
* <li>If there is a security manager, its {@code checkPermission} method is
* called to check {@code ReflectPermission("suppressAccessChecks")} and
* that must return normally.
* <li>The caller lookup object must have {@linkplain Lookup#hasFullPrivilegeAccess()
* full privilege access}. Specifically:
* <ul>
* <li>The caller lookup object must have the {@link Lookup#MODULE MODULE} lookup mode.
* (This is because otherwise there would be no way to ensure the original lookup
* creator was a member of any particular module, and so any subsequent checks
* for readability and qualified exports would become ineffective.)
* <li>The caller lookup object must have {@link Lookup#PRIVATE PRIVATE} access.
* (This is because an application intending to share intra-module access
* using {@link Lookup#MODULE MODULE} alone will inadvertently also share
* deep reflection to its own module.)
* </ul>
* <li>The target class must be a proper class, not a primitive or array class.
* (Thus, {@code M2} is well-defined.)
* <li>If the caller module {@code M1} differs from
* the target module {@code M2} then both of the following must be true:
* <ul>
* <li>{@code M1} {@link Module#canRead reads} {@code M2}.</li>
* <li>{@code M2} {@link Module#isOpen(String,Module) opens} the package
* containing the target class to at least {@code M1}.</li>
* </ul>
* </ul>
* <p>
* If any of the above checks is violated, this method fails with an
* exception.
* <p>
* Otherwise, if {@code M1} and {@code M2} are the same module, this method
* returns a {@code Lookup} on {@code targetClass} with
* {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}
* with {@code null} previous lookup class.
* <p>
* Otherwise, {@code M1} and {@code M2} are two different modules. This method
* returns a {@code Lookup} on {@code targetClass} that records
* the lookup class of the caller as the new previous lookup class with
* {@code PRIVATE} access but no {@code MODULE} access.
* <p>
* The resulting {@code Lookup} object has no {@code ORIGINAL} access.
*
* @param targetClass the target class
* @param caller the caller lookup object
* @return a lookup object for the target class, with private access
* @throws IllegalArgumentException if {@code targetClass} is a primitive type or void or array class
* @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
* @throws SecurityException if denied by the security manager
* @throws IllegalAccessException if any of the other access checks specified above fails
* @since 9
* @see Lookup#dropLookupMode
* @see <a href="MethodHandles.Lookup.html#cross-module-lookup">Cross-module lookups</a>
*/
public static Lookup privateLookupIn(Class<?> targetClass, Lookup caller) throws IllegalAccessException {
if (caller.allowedModes == Lookup.TRUSTED) {
return new Lookup(targetClass);
}
@SuppressWarnings("removal")
SecurityManager sm = System.getSecurityManager();
if (sm != null) sm.checkPermission(SecurityConstants.ACCESS_PERMISSION);
if (targetClass.isPrimitive())
throw new IllegalArgumentException(targetClass + " is a primitive class");
if (targetClass.isArray())
throw new IllegalArgumentException(targetClass + " is an array class");
// Ensure that we can reason accurately about private and module access.
int requireAccess = Lookup.PRIVATE|Lookup.MODULE;
if ((caller.lookupModes() & requireAccess) != requireAccess)
throw new IllegalAccessException("caller does not have PRIVATE and MODULE lookup mode");
// previous lookup class is never set if it has MODULE access
assert caller.previousLookupClass() == null;
Class<?> callerClass = caller.lookupClass();
Module callerModule = callerClass.getModule(); // M1
Module targetModule = targetClass.getModule(); // M2
Class<?> newPreviousClass = null;
int newModes = Lookup.FULL_POWER_MODES & ~Lookup.ORIGINAL;
if (targetModule != callerModule) {
if (!callerModule.canRead(targetModule))
throw new IllegalAccessException(callerModule + " does not read " + targetModule);
if (targetModule.isNamed()) {
String pn = targetClass.getPackageName();
assert !pn.isEmpty() : "unnamed package cannot be in named module";
if (!targetModule.isOpen(pn, callerModule))
throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
}
// M2 != M1, set previous lookup class to M1 and drop MODULE access
newPreviousClass = callerClass;
newModes &= ~Lookup.MODULE;
}
return Lookup.newLookup(targetClass, newPreviousClass, newModes);
}
/**
* Returns the <em>class data</em> associated with the lookup class
* of the given {@code caller} lookup object, or {@code null}.
*
* <p> A hidden class with class data can be created by calling
* {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
* Lookup::defineHiddenClassWithClassData}.
* This method will cause the static class initializer of the lookup
* class of the given {@code caller} lookup object be executed if
* it has not been initialized.
*
* <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
* Lookup::defineHiddenClass} and non-hidden classes have no class data.
* {@code null} is returned if this method is called on the lookup object
* on these classes.
*
* <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
* must have {@linkplain Lookup#ORIGINAL original access}
* in order to retrieve the class data.
*
* @apiNote
* This method can be called as a bootstrap method for a dynamically computed
* constant. A framework can create a hidden class with class data, for
* example that can be {@code Class} or {@code MethodHandle} object.
* The class data is accessible only to the lookup object
* created by the original caller but inaccessible to other members
* in the same nest. If a framework passes security sensitive objects
* to a hidden class via class data, it is recommended to load the value
* of class data as a dynamically computed constant instead of storing
* the class data in private static field(s) which are accessible to
* other nestmates.
*
* @param <T> the type to cast the class data object to
* @param caller the lookup context describing the class performing the
* operation (normally stacked by the JVM)
* @param name must be {@link ConstantDescs#DEFAULT_NAME}
* ({@code "_"})
* @param type the type of the class data
* @return the value of the class data if present in the lookup class;
* otherwise {@code null}
* @throws IllegalArgumentException if name is not {@code "_"}
* @throws IllegalAccessException if the lookup context does not have
* {@linkplain Lookup#ORIGINAL original} access
* @throws ClassCastException if the class data cannot be converted to
* the given {@code type}
* @throws NullPointerException if {@code caller} or {@code type} argument
* is {@code null}
* @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
* @see MethodHandles#classDataAt(Lookup, String, Class, int)
* @since 16
* @jvms 5.5 Initialization
*/
public static <T> T classData(Lookup caller, String name, Class<T> type) throws IllegalAccessException {
Objects.requireNonNull(caller);
Objects.requireNonNull(type);
if (!ConstantDescs.DEFAULT_NAME.equals(name)) {
throw new IllegalArgumentException("name must be \"_\": " + name);
}
if ((caller.lookupModes() & Lookup.ORIGINAL) != Lookup.ORIGINAL) {
throw new IllegalAccessException(caller + " does not have ORIGINAL access");
}
Object classdata = MethodHandleNatives.classData(caller.lookupClass());
if (classdata == null) return null;
try {
return BootstrapMethodInvoker.widenAndCast(classdata, type);
} catch (RuntimeException|Error e) {
throw e; // let CCE and other runtime exceptions through
} catch (Throwable e) {
throw new InternalError(e);
}
}
/**
* Returns the element at the specified index in the
* {@linkplain #classData(Lookup, String, Class) class data},
* if the class data associated with the lookup class
* of the given {@code caller} lookup object is a {@code List}.
* If the class data is not present in this lookup class, this method
* returns {@code null}.
*
* <p> A hidden class with class data can be created by calling
* {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
* Lookup::defineHiddenClassWithClassData}.
* This method will cause the static class initializer of the lookup
* class of the given {@code caller} lookup object be executed if
* it has not been initialized.
*
* <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
* Lookup::defineHiddenClass} and non-hidden classes have no class data.
* {@code null} is returned if this method is called on the lookup object
* on these classes.
*
* <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
* must have {@linkplain Lookup#ORIGINAL original access}
* in order to retrieve the class data.
*
* @apiNote
* This method can be called as a bootstrap method for a dynamically computed
* constant. A framework can create a hidden class with class data, for
* example that can be {@code List.of(o1, o2, o3....)} containing more than
* one object and use this method to load one element at a specific index.
* The class data is accessible only to the lookup object
* created by the original caller but inaccessible to other members
* in the same nest. If a framework passes security sensitive objects
* to a hidden class via class data, it is recommended to load the value
* of class data as a dynamically computed constant instead of storing
* the class data in private static field(s) which are accessible to other
* nestmates.
*
* @param <T> the type to cast the result object to
* @param caller the lookup context describing the class performing the
* operation (normally stacked by the JVM)
* @param name must be {@link java.lang.constant.ConstantDescs#DEFAULT_NAME}
* ({@code "_"})
* @param type the type of the element at the given index in the class data
* @param index index of the element in the class data
* @return the element at the given index in the class data
* if the class data is present; otherwise {@code null}
* @throws IllegalArgumentException if name is not {@code "_"}
* @throws IllegalAccessException if the lookup context does not have
* {@linkplain Lookup#ORIGINAL original} access
* @throws ClassCastException if the class data cannot be converted to {@code List}
* or the element at the specified index cannot be converted to the given type
* @throws IndexOutOfBoundsException if the index is out of range
* @throws NullPointerException if {@code caller} or {@code type} argument is
* {@code null}; or if unboxing operation fails because
* the element at the given index is {@code null}
*
* @since 16
* @see #classData(Lookup, String, Class)
* @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
*/
public static <T> T classDataAt(Lookup caller, String name, Class<T> type, int index)
throws IllegalAccessException
{
@SuppressWarnings("unchecked")
List<Object> classdata = (List<Object>)classData(caller, name, List.class);
if (classdata == null) return null;
try {
Object element = classdata.get(index);
return BootstrapMethodInvoker.widenAndCast(element, type);
} catch (RuntimeException|Error e) {
throw e; // let specified exceptions and other runtime exceptions/errors through
} catch (Throwable e) {
throw new InternalError(e);
}
}
/**
* Performs an unchecked "crack" of a
* <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
* The result is as if the user had obtained a lookup object capable enough
* to crack the target method handle, called
* {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
* on the target to obtain its symbolic reference, and then called
* {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
* to resolve the symbolic reference to a member.
* <p>
* If there is a security manager, its {@code checkPermission} method
* is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
* @param <T> the desired type of the result, either {@link Member} or a subtype
* @param target a direct method handle to crack into symbolic reference components
* @param expected a class object representing the desired result type {@code T}
* @return a reference to the method, constructor, or field object
* @throws SecurityException if the caller is not privileged to call {@code setAccessible}
* @throws NullPointerException if either argument is {@code null}
* @throws IllegalArgumentException if the target is not a direct method handle
* @throws ClassCastException if the member is not of the expected type
* @since 1.8
*/
public static <T extends Member> T reflectAs(Class<T> expected, MethodHandle target) {
@SuppressWarnings("removal")
SecurityManager smgr = System.getSecurityManager();
if (smgr != null) smgr.checkPermission(SecurityConstants.ACCESS_PERMISSION);
Lookup lookup = Lookup.IMPL_LOOKUP; // use maximally privileged lookup
return lookup.revealDirect(target).reflectAs(expected, lookup);
}
/**
* A <em>lookup object</em> is a factory for creating method handles,
* when the creation requires access checking.
* Method handles do not perform
* access checks when they are called, but rather when they are created.
* Therefore, method handle access
* restrictions must be enforced when a method handle is created.
* The caller class against which those restrictions are enforced
* is known as the {@linkplain #lookupClass() lookup class}.
* <p>
* A lookup class which needs to create method handles will call
* {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
* When the {@code Lookup} factory object is created, the identity of the lookup class is
* determined, and securely stored in the {@code Lookup} object.
* The lookup class (or its delegates) may then use factory methods
* on the {@code Lookup} object to create method handles for access-checked members.
* This includes all methods, constructors, and fields which are allowed to the lookup class,
* even private ones.
*
* <h2><a id="lookups"></a>Lookup Factory Methods</h2>
* The factory methods on a {@code Lookup} object correspond to all major
* use cases for methods, constructors, and fields.
* Each method handle created by a factory method is the functional
* equivalent of a particular <em>bytecode behavior</em>.
* (Bytecode behaviors are described in section {@jvms 5.4.3.5} of
* the Java Virtual Machine Specification.)
* Here is a summary of the correspondence between these factory methods and
* the behavior of the resulting method handles:
* <table class="striped">
* <caption style="display:none">lookup method behaviors</caption>
* <thead>
* <tr>
* <th scope="col"><a id="equiv"></a>lookup expression</th>
* <th scope="col">member</th>
* <th scope="col">bytecode behavior</th>
* </tr>
* </thead>
* <tbody>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
* <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
* <td>{@code static}<br>{@code FT f;}</td><td>{@code (FT) C.f;}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
* <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
* <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
* <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
* <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
* <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
* <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
* <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
* <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
* <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
* <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSpecial lookup.unreflectSpecial(aMethod,this.class)}</th>
* <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
* </tr>
* <tr>
* <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
* <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
* </tr>
* </tbody>
* </table>
*
* Here, the type {@code C} is the class or interface being searched for a member,
* documented as a parameter named {@code refc} in the lookup methods.
* The method type {@code MT} is composed from the return type {@code T}
* and the sequence of argument types {@code A*}.
* The constructor also has a sequence of argument types {@code A*} and
* is deemed to return the newly-created object of type {@code C}.
* Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
* The formal parameter {@code this} stands for the self-reference of type {@code C};
* if it is present, it is always the leading argument to the method handle invocation.
* (In the case of some {@code protected} members, {@code this} may be
* restricted in type to the lookup class; see below.)
* The name {@code arg} stands for all the other method handle arguments.
* In the code examples for the Core Reflection API, the name {@code thisOrNull}
* stands for a null reference if the accessed method or field is static,
* and {@code this} otherwise.
* The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
* for reflective objects corresponding to the given members declared in type {@code C}.
* <p>
* The bytecode behavior for a {@code findClass} operation is a load of a constant class,
* as if by {@code ldc CONSTANT_Class}.
* The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
* <p>
* In cases where the given member is of variable arity (i.e., a method or constructor)
* the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
* In all other cases, the returned method handle will be of fixed arity.
* <p style="font-size:smaller;">
* <em>Discussion:</em>
* The equivalence between looked-up method handles and underlying
* class members and bytecode behaviors
* can break down in a few ways:
* <ul style="font-size:smaller;">
* <li>If {@code C} is not symbolically accessible from the lookup class's loader,
* the lookup can still succeed, even when there is no equivalent
* Java expression or bytecoded constant.
* <li>Likewise, if {@code T} or {@code MT}
* is not symbolically accessible from the lookup class's loader,
* the lookup can still succeed.
* For example, lookups for {@code MethodHandle.invokeExact} and
* {@code MethodHandle.invoke} will always succeed, regardless of requested type.
* <li>If there is a security manager installed, it can forbid the lookup
* on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
* By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
* constant is not subject to security manager checks.
* <li>If the looked-up method has a
* <a href="MethodHandle.html#maxarity">very large arity</a>,
* the method handle creation may fail with an
* {@code IllegalArgumentException}, due to the method handle type having
* <a href="MethodHandle.html#maxarity">too many parameters.</a>
* </ul>
*
* <h2><a id="access"></a>Access checking</h2>
* Access checks are applied in the factory methods of {@code Lookup},
* when a method handle is created.
* This is a key difference from the Core Reflection API, since
* {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
* performs access checking against every caller, on every call.
* <p>
* All access checks start from a {@code Lookup} object, which
* compares its recorded lookup class against all requests to
* create method handles.
* A single {@code Lookup} object can be used to create any number
* of access-checked method handles, all checked against a single
* lookup class.
* <p>
* A {@code Lookup} object can be shared with other trusted code,
* such as a metaobject protocol.
* A shared {@code Lookup} object delegates the capability
* to create method handles on private members of the lookup class.
* Even if privileged code uses the {@code Lookup} object,
* the access checking is confined to the privileges of the
* original lookup class.
* <p>
* A lookup can fail, because
* the containing class is not accessible to the lookup class, or
* because the desired class member is missing, or because the
* desired class member is not accessible to the lookup class, or
* because the lookup object is not trusted enough to access the member.
* In the case of a field setter function on a {@code final} field,
* finality enforcement is treated as a kind of access control,
* and the lookup will fail, except in special cases of
* {@link Lookup#unreflectSetter Lookup.unreflectSetter}.
* In any of these cases, a {@code ReflectiveOperationException} will be
* thrown from the attempted lookup. The exact class will be one of
* the following:
* <ul>
* <li>NoSuchMethodException — if a method is requested but does not exist
* <li>NoSuchFieldException — if a field is requested but does not exist
* <li>IllegalAccessException — if the member exists but an access check fails
* </ul>
* <p>
* In general, the conditions under which a method handle may be
* looked up for a method {@code M} are no more restrictive than the conditions
* under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
* Where the JVM would raise exceptions like {@code NoSuchMethodError},
* a method handle lookup will generally raise a corresponding
* checked exception, such as {@code NoSuchMethodException}.
* And the effect of invoking the method handle resulting from the lookup
* is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
* to executing the compiled, verified, and resolved call to {@code M}.
* The same point is true of fields and constructors.
* <p style="font-size:smaller;">
* <em>Discussion:</em>
* Access checks only apply to named and reflected methods,
* constructors, and fields.
* Other method handle creation methods, such as
* {@link MethodHandle#asType MethodHandle.asType},
* do not require any access checks, and are used
* independently of any {@code Lookup} object.
* <p>
* If the desired member is {@code protected}, the usual JVM rules apply,
* including the requirement that the lookup class must either be in the
* same package as the desired member, or must inherit that member.
* (See the Java Virtual Machine Specification, sections {@jvms
* 4.9.2}, {@jvms 5.4.3.5}, and {@jvms 6.4}.)
* In addition, if the desired member is a non-static field or method
* in a different package, the resulting method handle may only be applied
* to objects of the lookup class or one of its subclasses.
* This requirement is enforced by narrowing the type of the leading
* {@code this} parameter from {@code C}
* (which will necessarily be a superclass of the lookup class)
* to the lookup class itself.
* <p>
* The JVM imposes a similar requirement on {@code invokespecial} instruction,
* that the receiver argument must match both the resolved method <em>and</em>
* the current class. Again, this requirement is enforced by narrowing the
* type of the leading parameter to the resulting method handle.
* (See the Java Virtual Machine Specification, section {@jvms 4.10.1.9}.)
* <p>
* The JVM represents constructors and static initializer blocks as internal methods
* with special names ({@code "<init>"} and {@code "<clinit>"}).
* The internal syntax of invocation instructions allows them to refer to such internal
* methods as if they were normal methods, but the JVM bytecode verifier rejects them.
* A lookup of such an internal method will produce a {@code NoSuchMethodException}.
* <p>
* If the relationship between nested types is expressed directly through the
* {@code NestHost} and {@code NestMembers} attributes
* (see the Java Virtual Machine Specification, sections {@jvms
* 4.7.28} and {@jvms 4.7.29}),
* then the associated {@code Lookup} object provides direct access to
* the lookup class and all of its nestmates
* (see {@link java.lang.Class#getNestHost Class.getNestHost}).
* Otherwise, access between nested classes is obtained by the Java compiler creating
* a wrapper method to access a private method of another class in the same nest.
* For example, a nested class {@code C.D}
* can access private members within other related classes such as
* {@code C}, {@code C.D.E}, or {@code C.B},
* but the Java compiler may need to generate wrapper methods in
* those related classes. In such cases, a {@code Lookup} object on
* {@code C.E} would be unable to access those private members.
* A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
* which can transform a lookup on {@code C.E} into one on any of those other
* classes, without special elevation of privilege.
* <p>
* The accesses permitted to a given lookup object may be limited,
* according to its set of {@link #lookupModes lookupModes},
* to a subset of members normally accessible to the lookup class.
* For example, the {@link MethodHandles#publicLookup publicLookup}
* method produces a lookup object which is only allowed to access
* public members in public classes of exported packages.
* The caller sensitive method {@link MethodHandles#lookup lookup}
* produces a lookup object with full capabilities relative to
* its caller class, to emulate all supported bytecode behaviors.
* Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
* with fewer access modes than the original lookup object.
*
* <p style="font-size:smaller;">
* <a id="privacc"></a>
* <em>Discussion of private and module access:</em>
* We say that a lookup has <em>private access</em>
* if its {@linkplain #lookupModes lookup modes}
* include the possibility of accessing {@code private} members
* (which includes the private members of nestmates).
* As documented in the relevant methods elsewhere,
* only lookups with private access possess the following capabilities:
* <ul style="font-size:smaller;">
* <li>access private fields, methods, and constructors of the lookup class and its nestmates
* <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
* <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
* for classes accessible to the lookup class
* <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
* within the same package member
* </ul>
* <p style="font-size:smaller;">
* Similarly, a lookup with module access ensures that the original lookup creator was
* a member in the same module as the lookup class.
* <p style="font-size:smaller;">
* Private and module access are independently determined modes; a lookup may have
* either or both or neither. A lookup which possesses both access modes is said to
* possess {@linkplain #hasFullPrivilegeAccess() full privilege access}.
* <p style="font-size:smaller;">
* A lookup with <em>original access</em> ensures that this lookup is created by
* the original lookup class and the bootstrap method invoked by the VM.
* Such a lookup with original access also has private and module access
* which has the following additional capability:
* <ul style="font-size:smaller;">
* <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
* such as {@code Class.forName}
* <li>obtain the {@linkplain MethodHandles#classData(Lookup, String, Class)
* class data} associated with the lookup class</li>
* </ul>
* <p style="font-size:smaller;">
* Each of these permissions is a consequence of the fact that a lookup object
* with private access can be securely traced back to an originating class,
* whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
* can be reliably determined and emulated by method handles.
*
* <h2><a id="cross-module-lookup"></a>Cross-module lookups</h2>
* When a lookup class in one module {@code M1} accesses a class in another module
* {@code M2}, extra access checking is performed beyond the access mode bits.
* A {@code Lookup} with {@link #PUBLIC} mode and a lookup class in {@code M1}
* can access public types in {@code M2} when {@code M2} is readable to {@code M1}
* and when the type is in a package of {@code M2} that is exported to
* at least {@code M1}.
* <p>
* A {@code Lookup} on {@code C} can also <em>teleport</em> to a target class
* via {@link #in(Class) Lookup.in} and {@link MethodHandles#privateLookupIn(Class, Lookup)
* MethodHandles.privateLookupIn} methods.
* Teleporting across modules will always record the original lookup class as
* the <em>{@linkplain #previousLookupClass() previous lookup class}</em>
* and drops {@link Lookup#MODULE MODULE} access.
* If the target class is in the same module as the lookup class {@code C},
* then the target class becomes the new lookup class
* and there is no change to the previous lookup class.
* If the target class is in a different module from {@code M1} ({@code C}'s module),
* {@code C} becomes the new previous lookup class
* and the target class becomes the new lookup class.
* In that case, if there was already a previous lookup class in {@code M0},
* and it differs from {@code M1} and {@code M2}, then the resulting lookup
* drops all privileges.
* For example,
* <blockquote><pre>
* {@code
* Lookup lookup = MethodHandles.lookup(); // in class C
* Lookup lookup2 = lookup.in(D.class);
* MethodHandle mh = lookup2.findStatic(E.class, "m", MT);
* }</pre></blockquote>
* <p>
* The {@link #lookup()} factory method produces a {@code Lookup} object
* with {@code null} previous lookup class.
* {@link Lookup#in lookup.in(D.class)} transforms the {@code lookup} on class {@code C}
* to class {@code D} without elevation of privileges.
* If {@code C} and {@code D} are in the same module,
* {@code lookup2} records {@code D} as the new lookup class and keeps the
* same previous lookup class as the original {@code lookup}, or
* {@code null} if not present.
* <p>
* When a {@code Lookup} teleports from a class
* in one nest to another nest, {@code PRIVATE} access is dropped.
* When a {@code Lookup} teleports from a class in one package to
* another package, {@code PACKAGE} access is dropped.
* When a {@code Lookup} teleports from a class in one module to another module,
* {@code MODULE} access is dropped.
* Teleporting across modules drops the ability to access non-exported classes
* in both the module of the new lookup class and the module of the old lookup class
* and the resulting {@code Lookup} remains only {@code PUBLIC} access.
* A {@code Lookup} can teleport back and forth to a class in the module of
* the lookup class and the module of the previous class lookup.
* Teleporting across modules can only decrease access but cannot increase it.
* Teleporting to some third module drops all accesses.
* <p>
* In the above example, if {@code C} and {@code D} are in different modules,
* {@code lookup2} records {@code D} as its lookup class and
* {@code C} as its previous lookup class and {@code lookup2} has only
* {@code PUBLIC} access. {@code lookup2} can teleport to other class in
* {@code C}'s module and {@code D}'s module.
* If class {@code E} is in a third module, {@code lookup2.in(E.class)} creates
* a {@code Lookup} on {@code E} with no access and {@code lookup2}'s lookup
* class {@code D} is recorded as its previous lookup class.
* <p>
* Teleporting across modules restricts access to the public types that
* both the lookup class and the previous lookup class can equally access
* (see below).
* <p>
* {@link MethodHandles#privateLookupIn(Class, Lookup) MethodHandles.privateLookupIn(T.class, lookup)}
* can be used to teleport a {@code lookup} from class {@code C} to class {@code T}
* and create a new {@code Lookup} with <a href="#privacc">private access</a>
* if the lookup class is allowed to do <em>deep reflection</em> on {@code T}.
* The {@code lookup} must have {@link #MODULE} and {@link #PRIVATE} access
* to call {@code privateLookupIn}.
* A {@code lookup} on {@code C} in module {@code M1} is allowed to do deep reflection
* on all classes in {@code M1}. If {@code T} is in {@code M1}, {@code privateLookupIn}
* produces a new {@code Lookup} on {@code T} with full capabilities.
* A {@code lookup} on {@code C} is also allowed
* to do deep reflection on {@code T} in another module {@code M2} if
* {@code M1} reads {@code M2} and {@code M2} {@link Module#isOpen(String,Module) opens}
* the package containing {@code T} to at least {@code M1}.
* {@code T} becomes the new lookup class and {@code C} becomes the new previous
* lookup class and {@code MODULE} access is dropped from the resulting {@code Lookup}.
* The resulting {@code Lookup} can be used to do member lookup or teleport
* to another lookup class by calling {@link #in Lookup::in}. But
* it cannot be used to obtain another private {@code Lookup} by calling
* {@link MethodHandles#privateLookupIn(Class, Lookup) privateLookupIn}
* because it has no {@code MODULE} access.
*
* <h2><a id="module-access-check"></a>Cross-module access checks</h2>
*
* A {@code Lookup} with {@link #PUBLIC} or with {@link #UNCONDITIONAL} mode
* allows cross-module access. The access checking is performed with respect
* to both the lookup class and the previous lookup class if present.
* <p>
* A {@code Lookup} with {@link #UNCONDITIONAL} mode can access public type
* in all modules when the type is in a package that is {@linkplain Module#isExported(String)
* exported unconditionally}.
* <p>
* If a {@code Lookup} on {@code LC} in {@code M1} has no previous lookup class,
* the lookup with {@link #PUBLIC} mode can access all public types in modules
* that are readable to {@code M1} and the type is in a package that is exported
* at least to {@code M1}.
* <p>
* If a {@code Lookup} on {@code LC} in {@code M1} has a previous lookup class
* {@code PLC} on {@code M0}, the lookup with {@link #PUBLIC} mode can access
* the intersection of all public types that are accessible to {@code M1}
* with all public types that are accessible to {@code M0}. {@code M0}
* reads {@code M1} and hence the set of accessible types includes:
*
* <table class="striped">
* <caption style="display:none">
* Public types in the following packages are accessible to the
* lookup class and the previous lookup class.
* </caption>
* <thead>
* <tr>
* <th scope="col">Equally accessible types to {@code M0} and {@code M1}</th>
* </tr>
* </thead>
* <tbody>
* <tr>
* <th scope="row" style="text-align:left">unconditional-exported packages from {@code M1}</th>
* </tr>
* <tr>
* <th scope="row" style="text-align:left">unconditional-exported packages from {@code M0} if {@code M1} reads {@code M0}</th>
* </tr>
* <tr>
* <th scope="row" style="text-align:left">unconditional-exported packages from a third module {@code M2}
* if both {@code M0} and {@code M1} read {@code M2}</th>
* </tr>
* <tr>
* <th scope="row" style="text-align:left">qualified-exported packages from {@code M1} to {@code M0}</th>
* </tr>
* <tr>
* <th scope="row" style="text-align:left">qualified-exported packages from {@code M0} to {@code M1}
* if {@code M1} reads {@code M0}</th>
* </tr>
* <tr>
* <th scope="row" style="text-align:left">qualified-exported packages from a third module {@code M2} to
* both {@code M0} and {@code M1} if both {@code M0} and {@code M1} read {@code M2}</th>
* </tr>
* </tbody>
* </table>
*
* <h2><a id="access-modes"></a>Access modes</h2>
*
* The table below shows the access modes of a {@code Lookup} produced by
* any of the following factory or transformation methods:
* <ul>
* <li>{@link #lookup() MethodHandles::lookup}</li>
* <li>{@link #publicLookup() MethodHandles::publicLookup}</li>
* <li>{@link #privateLookupIn(Class, Lookup) MethodHandles::privateLookupIn}</li>
* <li>{@link Lookup#in Lookup::in}</li>
* <li>{@link Lookup#dropLookupMode(int) Lookup::dropLookupMode}</li>
* </ul>
*
* <table class="striped">
* <caption style="display:none">
* Access mode summary
* </caption>
* <thead>
* <tr>
* <th scope="col">Lookup object</th>
* <th style="text-align:center">original</th>
* <th style="text-align:center">protected</th>
* <th style="text-align:center">private</th>
* <th style="text-align:center">package</th>
* <th style="text-align:center">module</th>
* <th style="text-align:center">public</th>
* </tr>
* </thead>
* <tbody>
* <tr>
* <th scope="row" style="text-align:left">{@code CL = MethodHandles.lookup()} in {@code C}</th>
* <td style="text-align:center">ORI</td>
* <td style="text-align:center">PRO</td>
* <td style="text-align:center">PRI</td>
* <td style="text-align:center">PAC</td>
* <td style="text-align:center">MOD</td>
* <td style="text-align:center">1R</td>
* </tr>
* <tr>
* <th scope="row" style="text-align:left">{@code CL.in(C1)} same package</th>
* <td></td>
* <td></td>
* <td></td>
* <td style="text-align:center">PAC</td>
* <td style="text-align:center">MOD</td>
* <td style="text-align:center">1R</td>
* </tr>
* <tr>
* <th scope="row" style="text-align:left">{@code CL.in(C1)} same module</th>
* <td></td>
* <td></td>
* <td></td>
* <td></td>
* <td style="text-align:center">MOD</td>
* <td style="text-align:center">1R</td>
* </tr>
* <tr>
* <th scope="row" style="text-align:left">{@code CL.in(D)} different module</th>
* <td></td>
* <td></td>
* <td></td>
* <td></td>
* <td></td>
* <td style="text-align:center">2R</td>
* </tr>
* <tr>
* <td>{@code CL.in(D).in(C)} hop back to module</td>
* <td></td>
* <td></td>
* <td></td>
* <td></td>
* <td></td>
* <td style="text-align:center">2R</td>
* </tr>
* <tr>
* <td>{@code PRI1 = privateLookupIn(C1,CL)}</td>
* <td></td>
* <td style="text-align:center">PRO</td>
* <td style="text-align:center">PRI</td>
* <td style="text-align:center">PAC</td>
* <td style="text-align:center">MOD</td>
* <td style="text-align:center">1R</td>
* </tr>
* <tr>
* <td>{@code PRI1a = privateLookupIn(C,PRI1)}</td>
* <td></td>
* <td style="text-align:center">PRO</td>
* <td style="text-align:center">PRI</td>
* <td style="text-align:center">PAC</td>
* <td style="text-align:center">MOD</td>
* <td style="text-align:center">1R</td>
* </tr>
* <tr>
* <td>{@code PRI1.in(C1)} same package</td>
* <td></td>
* <td></td>
* <td></td>
* <td style="text-align:center">PAC</td>
* <td style="text-align:center">MOD</td>
* <td style="text-align:center">1R</td>
* </tr>
* <tr>
* <td>{@code PRI1.in(C1)} different package</td>
* <td></td>