InvocationHandler.java

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package java.lang.reflect;

import jdk.internal.reflect.CallerSensitive;
import jdk.internal.reflect.Reflection;

import java.util.Objects;

/**
 * {@code InvocationHandler} is the interface implemented by
 * the <i>invocation handler</i> of a proxy instance.
 *
 * <p>Each proxy instance has an associated invocation handler.
 * When a method is invoked on a proxy instance, the method
 * invocation is encoded and dispatched to the {@code invoke}
 * method of its invocation handler.
 *
 * @author      Peter Jones
 * @see         Proxy
 * @since       1.3
 */
public interface InvocationHandler {

    /**
     * Processes a method invocation on a proxy instance and returns
     * the result.  This method will be invoked on an invocation handler
     * when a method is invoked on a proxy instance that it is
     * associated with.
     *
     * @param   proxy the proxy instance that the method was invoked on
     *
     * @param   method the {@code Method} instance corresponding to
     * the interface method invoked on the proxy instance.  The declaring
     * class of the {@code Method} object will be the interface that
     * the method was declared in, which may be a superinterface of the
     * proxy interface that the proxy class inherits the method through.
     *
     * @param   args an array of objects containing the values of the
     * arguments passed in the method invocation on the proxy instance,
     * or {@code null} if interface method takes no arguments.
     * Arguments of primitive types are wrapped in instances of the
     * appropriate primitive wrapper class, such as
     * {@code java.lang.Integer} or {@code java.lang.Boolean}.
     *
     * @return  the value to return from the method invocation on the
     * proxy instance.  If the declared return type of the interface
     * method is a primitive type, then the value returned by
     * this method must be an instance of the corresponding primitive
     * wrapper class; otherwise, it must be a type assignable to the
     * declared return type.  If the value returned by this method is
     * {@code null} and the interface method's return type is
     * primitive, then a {@code NullPointerException} will be
     * thrown by the method invocation on the proxy instance.  If the
     * value returned by this method is otherwise not compatible with
     * the interface method's declared return type as described above,
     * a {@code ClassCastException} will be thrown by the method
     * invocation on the proxy instance.
     *
     * @throws  Throwable the exception to throw from the method
     * invocation on the proxy instance.  The exception's type must be
     * assignable either to any of the exception types declared in the
     * {@code throws} clause of the interface method or to the
     * unchecked exception types {@code java.lang.RuntimeException}
     * or {@code java.lang.Error}.  If a checked exception is
     * thrown by this method that is not assignable to any of the
     * exception types declared in the {@code throws} clause of
     * the interface method, then an
     * {@link UndeclaredThrowableException} containing the
     * exception that was thrown by this method will be thrown by the
     * method invocation on the proxy instance.
     *
     * @see     UndeclaredThrowableException
     */
    public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable;

    /**
     * Invokes the specified default method on the given {@code proxy} instance with
     * the given parameters.  The given {@code method} must be a default method
     * declared in a proxy interface of the {@code proxy}'s class or inherited
     * from its superinterface directly or indirectly.
     * <p>
     * Invoking this method behaves as if {@code invokespecial} instruction executed
     * from the proxy class, targeting the default method in a proxy interface.
     * This is equivalent to the invocation:
     * {@code X.super.m(A* a)} where {@code X} is a proxy interface and the call to
     * {@code X.super::m(A*)} is resolved to the given {@code method}.
     * <p>
     * Examples: interface {@code A} and {@code B} both declare a default
     * implementation of method {@code m}. Interface {@code C} extends {@code A}
     * and inherits the default method {@code m} from its superinterface {@code A}.
     *
     * <blockquote><pre>{@code
     * interface A {
     *     default T m(A a) { return t1; }
     * }
     * interface B {
     *     default T m(A a) { return t2; }
     * }
     * interface C extends A {}
     * }</pre></blockquote>
     *
     * The following creates a proxy instance that implements {@code A}
     * and invokes the default method {@code A::m}.
     *
     * <blockquote><pre>{@code
     * Object proxy = Proxy.newProxyInstance(loader, new Class<?>[] { A.class },
     *         (o, m, params) -> {
     *             if (m.isDefault()) {
     *                 // if it's a default method, invoke it
     *                 return InvocationHandler.invokeDefault(o, m, params);
     *             }
     *         });
     * }</pre></blockquote>
     *
     * If a proxy instance implements both {@code A} and {@code B}, both
     * of which provides the default implementation of method {@code m},
     * the invocation handler can dispatch the method invocation to
     * {@code A::m} or {@code B::m} via the {@code invokeDefault} method.
     * For example, the following code delegates the method invocation
     * to {@code B::m}.
     *
     * <blockquote><pre>{@code
     * Object proxy = Proxy.newProxyInstance(loader, new Class<?>[] { A.class, B.class },
     *         (o, m, params) -> {
     *             if (m.getName().equals("m")) {
     *                 // invoke B::m instead of A::m
     *                 Method bMethod = B.class.getMethod(m.getName(), m.getParameterTypes());
     *                 return InvocationHandler.invokeDefault(o, bMethod, params);
     *             }
     *         });
     * }</pre></blockquote>
     *
     * If a proxy instance implements {@code C} that inherits the default
     * method {@code m} from its superinterface {@code A}, then
     * the interface method invocation on {@code "m"} is dispatched to
     * the invocation handler's {@link #invoke(Object, Method, Object[]) invoke}
     * method with the {@code Method} object argument representing the
     * default method {@code A::m}.
     *
     * <blockquote><pre>{@code
     * Object proxy = Proxy.newProxyInstance(loader, new Class<?>[] { C.class },
     *        (o, m, params) -> {
     *             if (m.isDefault()) {
     *                 // behaves as if calling C.super.m(params)
     *                 return InvocationHandler.invokeDefault(o, m, params);
     *             }
     *        });
     * }</pre></blockquote>
     *
     * The invocation of method {@code "m"} on this {@code proxy} will behave
     * as if {@code C.super::m} is called and that is resolved to invoking
     * {@code A::m}.
     * <p>
     * Adding a default method, or changing a method from abstract to default
     * may cause an exception if an existing code attempts to call {@code invokeDefault}
     * to invoke a default method.
     *
     * For example, if {@code C} is modified to implement a default method
     * {@code m}:
     *
     * <blockquote><pre>{@code
     * interface C extends A {
     *     default T m(A a) { return t3; }
     * }
     * }</pre></blockquote>
     *
     * The code above that creates proxy instance {@code proxy} with
     * the modified {@code C} will run with no exception and it will result in
     * calling {@code C::m} instead of {@code A::m}.
     * <p>
     * The following is another example that creates a proxy instance of {@code C}
     * and the invocation handler calls the {@code invokeDefault} method
     * to invoke {@code A::m}:
     *
     * <blockquote><pre>{@code
     * C c = (C) Proxy.newProxyInstance(loader, new Class<?>[] { C.class },
     *         (o, m, params) -> {
     *             if (m.getName().equals("m")) {
     *                 // IllegalArgumentException thrown as {@code A::m} is not a method
     *                 // inherited from its proxy interface C
     *                 Method aMethod = A.class.getMethod(m.getName(), m.getParameterTypes());
     *                 return InvocationHandler.invokeDefault(o, aMethod params);
     *             }
     *         });
     * c.m(...);
     * }</pre></blockquote>
     *
     * The above code runs successfully with the old version of {@code C} and
     * {@code A::m} is invoked.  When running with the new version of {@code C},
     * the above code will fail with {@code IllegalArgumentException} because
     * {@code C} overrides the implementation of the same method and
     * {@code A::m} is not accessible by a proxy instance.
     *
     * @apiNote
     * The {@code proxy} parameter is of type {@code Object} rather than {@code Proxy}
     * to make it easy for {@link InvocationHandler#invoke(Object, Method, Object[])
     * InvocationHandler::invoke} implementation to call directly without the need
     * of casting.
     *
     * @param proxy   the {@code Proxy} instance on which the default method to be invoked
     * @param method  the {@code Method} instance corresponding to a default method
     *                declared in a proxy interface of the proxy class or inherited
     *                from its superinterface directly or indirectly
     * @param args    the parameters used for the method invocation; can be {@code null}
     *                if the number of formal parameters required by the method is zero.
     * @return the value returned from the method invocation
     *
     * @throws IllegalArgumentException if any of the following conditions is {@code true}:
     *         <ul>
     *         <li>{@code proxy} is not {@linkplain Proxy#isProxyClass(Class)
     *             a proxy instance}; or</li>
     *         <li>the given {@code method} is not a default method declared
     *             in a proxy interface of the proxy class and not inherited from
     *             any of its superinterfaces; or</li>
     *         <li>the given {@code method} is overridden directly or indirectly by
     *             the proxy interfaces and the method reference to the named
     *             method never resolves to the given {@code method}; or</li>
     *         <li>the length of the given {@code args} array does not match the
     *             number of parameters of the method to be invoked; or</li>
     *         <li>any of the {@code args} elements fails the unboxing
     *             conversion if the corresponding method parameter type is
     *             a primitive type; or if, after possible unboxing, any of the
     *             {@code args} elements cannot be assigned to the corresponding
     *             method parameter type.</li>
     *         </ul>
     * @throws IllegalAccessException if the declaring class of the specified
     *         default method is inaccessible to the caller class
     * @throws NullPointerException if {@code proxy} or {@code method} is {@code null}
     * @throws Throwable anything thrown by the default method

     * @since 16
     * @jvms 5.4.3. Method Resolution
     */
    @CallerSensitive
    public static Object invokeDefault(Object proxy, Method method, Object... args)
            throws Throwable {
        Objects.requireNonNull(proxy);
        Objects.requireNonNull(method);
        return Proxy.invokeDefault(proxy, method, args, Reflection.getCallerClass());
    }
}