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Tutorial 2
CtClass provides methods for introspection. The
introspective ability of Javassist is compatible with that of
the Java reflection API. CtClass provides
getName(), getSuperclass(),
getMethods(), and so on.
CtClass also provides methods for modifying a class
definition. It allows adding a new field, constructor, and method.
Instrumenting a method body is also possible.
Methods are represented by CtMethod objects.
CtMethod provides several methods for modifying
the definition of the method. Note that if a method is inherited
from a super class, then
the same CtMethod object
that represents the inherited method represents the method declared
in that super class.
A CtMethod object corresponds to every method declaration.
For example, if class Point declares method move()
and a subclass ColorPoint of Point does
not override move(), the two move() methods
declared in Point and inherited in ColorPoint
are represented by the identical CtMethod object.
If the method definition represented by this
CtMethod object is modified, the modification is
reflected on both the methods.
If you want to modify only the move() method in
ColorPoint, you first have to add to ColorPoint
a copy of the CtMethod object representing move()
in Point. A copy of the the CtMethod object
can be obtained by CtNewMethod.copy().
-
Javassist does not allow removing a method or field, but it allows
changing the name. So if a method is not necessary any more, it should be
renamed and changed to be a private method by calling
setName()
and setModifiers() declared in CtMethod.
Javassist does not allow adding an extra parameter to an existing
method, either. Instead of doing that, a new method receiving the
extra parameter as well as the other parameters should be added to the
same class. For example, if you want to add an extra int
parameter newZ to a method:
void move(int newX, int newY) { x = newX; y = newY; }
in a Point class, then you should add the following
method to the Point class:
void move(int newX, int newY, int newZ) {
// do what you want with newZ.
move(newX, newY);
}
Javassist also provides low-level API for directly editing a raw
class file. For example, getClassFile() in
CtClass returns a ClassFile object
representing a raw class file. getMethodInfo() in
CtMethod returns a MethodInfo object
representing a method_info structure included in a class
file. The low-level API uses the vocabulary from the Java Virtual
machine specification. The users must have the knowledge about class
files and bytecode. For more details, the users should see the
javassist.bytecode package.
The class files modified by Javassist requires the
javassist.runtime package for runtime support
only if some special identifiers starting with $
are used. Those special identifiers are described below.
The class files modified without those special identifiers
do not need the javassist.runtime package or any
other Javassist packages at runtime.
For more details, see the API documentation
of the javassist.runtime package.
CtMethod and CtConstructor provide
methods insertBefore(), insertAfter(), and
addCatch(). They are used for inserting a code fragment
into the body of an existing method. The users can specify those code
fragments with source text written in Java.
Javassist includes a simple Java compiler for processing source
text. It receives source text
written in Java and compiles it into Java bytecode, which will be
inlined into a method body.
Inserting a code fragment at the position specified by a line number
is also possible
(if the line number table is contained in the class file).
insertAt() in CtMethod and
CtConstructor takes source text and a line number in the source
file of the original class definition.
It compiles the source text and inserts the compiled code at the line number.
The methods insertBefore(), insertAfter(),
addCatch(), and insertAt()
receive a String object representing
a statement or a block. A statement is a single control structure like
if and while or an expression ending with
a semi colon (;). A block is a set of
statements surrounded with braces {}.
Hence each of the following lines is an example of valid statement or block:
System.out.println("Hello");
{ System.out.println("Hello"); }
if (i < 0) { i = -i; }
The statement and the block can refer to fields and methods.
They can also refer to the parameters
to the method that they are inserted into
if that method was compiled with the -g option
(to include a local variable attribute in the class file).
Otherwise, they must access the method parameters through the special
variables $0, $1, $2, ... described
below.
Accessing local variables declared in the method is not allowed
although declaring a new local variable in the block is allowed.
However, insertAt() allows the statement and the block
to access local variables
if these variables are available at the specified line number
and the target method was compiled with the -g option.
The String object passed to the methods
insertBefore(), insertAfter(),
addCatch(), and insertAt() are compiled by
the compiler included in Javassist.
Since the compiler supports language extensions,
several identifiers starting with $
have special meaning:
$0, $1, $2, ...     |
this and actual parameters |
$args |
An array of parameters.
The type of $args is Object[].
|
$$ |
All actual parameters. For example, m($$) is equivalent to
m($1,$2,...)
|
|   | |
$cflow(...)
|
cflow variable |
$r |
The result type. It is used in a cast expression. |
$w |
The wrapper type. It is used in a cast expression. |
$_ |
The resulting value |
$sig |
An array of java.lang.Class objects representing
the formal parameter types.
|
$type |
A java.lang.Class object representing
the formal result type. |
$class |
A java.lang.Class object representing
the class currently edited. |
The parameters passed to the target method
are accessible with
$1, $2, ... instead of
the original parameter names.
$1 represents the
first parameter, $2 represents the second parameter, and
so on. The types of those variables are identical to the parameter
types.
$0 is
equivalent to this. If the method is static,
$0 is not available.
These variables are used as following. Suppose that a class
Point:
class Point {
int x, y;
void move(int dx, int dy) { x += dx; y += dy; }
}
To print the values of dx and dy
whenever the method move() is called, execute this
program:
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("Point");
CtMethod m = cc.getDeclaredMethod("move");
m.insertBefore("{ System.out.println($1); System.out.println($2); }");
cc.writeFile();
Note that the source text passed to insertBefore() is
surrounded with braces {}.
insertBefore() accepts only a single statement or a block
surrounded with braces.
The definition of the class Point after the
modification is like this:
class Point {
int x, y;
void move(int dx, int dy) {
{ System.out.println(dx); System.out.println(dy); }
x += dx; y += dy;
}
}
$1 and $2 are replaced with
dx and dy, respectively.
$1, $2, $3 ... are
updatable. If a new value is assigend to one of those variables,
then the value of the parameter represented by that variable is
also updated.
The variable $args represents an array of all the
parameters. The type of that variable is an array of class
Object. If a parameter type is a primitive type such as
int, then the parameter value is converted into a wrapper
object such as java.lang.Integer to store in
$args. Thus, $args[0] is equivalent to
$1 unless the type of the first parameter is a primitive
type. Note that $args[0] is not equivalent to
$0; $0 represents this.
If an array of Object is assigned to
$args, then each element of that array is
assigned to each parameter. If a parameter type is a primitive
type, the type of the corresponding element must be a wrapper type.
The value is converted from the wrapper type to the primitive type
before it is assigned to the parameter.
The variable $$ is abbreviation of a list of
all the parameters separated by commas.
For example, if the number of the parameters
to method move() is three, then
move($$)
is equivalent to this:
move($1, $2, $3)
If move() does not take any parameters,
then move($$) is
equivalent to move().
$$ can be used with another method.
If you write an expression:
exMove($$, context)
then this expression is equivalent to:
exMove($1, $2, $3, context)
Note that $$ enables generic notation of method call
with respect to the number of parameters.
It is typically used with $proceed shown later.
$cflow means "control flow".
This read-only variable returns the depth of the recursive calls
to a specific method.
Suppose that the method shown below is represented by a
CtMethod object cm:
int fact(int n) {
if (n <= 1)
return n;
else
return n * fact(n - 1);
}
To use $cflow, first declare that $cflow
is used for monitoring calls to the method fact():
CtMethod cm = ...;
cm.useCflow("fact");
The parameter to useCflow() is the identifier of the
declared $cflow variable. Any valid Java name can be
used as the identifier. Since the identifier can also include
. (dot), for example, "my.Test.fact"
is a valid identifier.
Then, $cflow(fact) represents the depth of the
recursive calls to the method specified by cm. The value
of $cflow(fact) is 0 (zero) when the method is
first called whereas it is 1 when the method is recursively called
within the method. For example,
cm.insertBefore("if ($cflow(fact) == 0)"
+ " System.out.println(\"fact \" + $1);");
translates the method fact() so that it shows the
parameter. Since the value of $cflow(fact) is checked,
the method fact() does not show the parameter if it is
recursively called within fact().
The value of $cflow is the number of stack frames
associated with the specified method cm
under the current topmost
stack frame for the current thread. $cflow is also
accessible within a method different from the specified method
cm.
$r represents the result type (return type) of the method.
It must be used as the cast type in a cast expression.
For example, this is a typical use:
Object result = ... ; $_ = ($r)result;
If the result type is a primitive type, then ($r)
follows special semantics. First, if the operand type of the cast
expression is a primitive type, ($r) works as a normal
cast operator to the result type.
On the other hand, if the operand type is a wrapper type,
($r) converts from the wrapper type to the result type.
For example, if the result type is int, then
($r) converts from java.lang.Integer to
int.
If the result type is void, then
($r) does not convert a type; it does nothing.
However, if the operand is a call to a void method,
then ($r) results in null. For example,
if the result type is void and
foo() is a void method, then
$_ = ($r)foo();
is a valid statement.
The cast operator ($r) is also useful in a
return statement. Even if the result type is
void, the following return statement is valid:
return ($r)result;
Here, result is some local variable.
Since ($r) is specified, the resulting value is
discarded.
This return statement is regarded as the equivalent
of the return statement without a resulting value:
return;
$w represents a wrapper type.
It must be used as the cast type in a cast expression.
($w) converts from a primitive type to the corresponding
wrapper type.
The following code is an example:
Integer i = ($w)5;
The selected wrapper type depends on the type of the expression
following ($w). If the type of the expression is
double, then the wrapper type is java.lang.Double.
If the type of the expression following ($w) is not
a primitive type, then ($w) does nothing.
insertAfter() in CtMethod and
CtConstructor inserts the
compiled code at the end of the method. In the statement given to
insertAfter(), not only the variables shown above such as
$0, $1, ... but also $_ is
available.
The variable $_ represents the resulting value of the
method. The type of that variable is the type of the result type (the
return type) of the method. If the result type is void,
then the type of $_ is Object and the value
of $_ is null.
Although the compiled code inserted by insertAfter()
is executed just before the control normally returns from the method,
it can be also executed when an exception is thrown from the method.
To execute it when an exception is thrown, the second parameter
asFinally to insertAfter() must be
true.
If an exception is thrown, the compiled code inserted by
insertAfter() is executed as a finally
clause. The value of $_ is 0 or
null in the compiled code. After the execution of the
compiled code terminates, the exception originally thrown is re-thrown
to the caller. Note that the value of $_ is never thrown
to the caller; it is rather discarded.
The value of $sig is an array of
java.lang.Class objects that represent the formal
parameter types in declaration order.
The value of $type is an java.lang.Class
object representing the formal type of the result value. This
variable refers to Void.class if this is a constructor.
The value of $class is an java.lang.Class
object representing the class in which the edited method is declared.
This represents the type of $0.
addCatch() inserts a code fragment into a method body
so that the code fragment is executed when the method body throws
an exception and the control returns to the caller. In the source
text representing the inserted code fragment, the exception value
is referred to with the special variable $e.
For example, this program:
CtMethod m = ...;
CtClass etype = ClassPool.getDefault().get("java.io.IOException");
m.addCatch("{ System.out.println($e); throw $e; }", etype);
translates the method body represented by m into
something like this:
try {
the original method body
}
catch (java.io.IOException e) {
System.out.println(e);
throw e;
}
Note that the inserted code fragment must end with a
throw or return statement.
CtMethod and CtConstructor provide
setBody() for substituting a whole
method body. They compile the given source text into Java bytecode
and substitutes it for the original method body. If the given source
text is null, the substituted body includes only a
return statement, which returns zero or null unless the
result type is void.
In the source text given to setBody(), the identifiers
starting with $ have special meaning
$0, $1, $2, ... |
this and actual parameters |
$args |
An array of parameters.
The type of $args is Object[].
|
$$ |
All actual parameters. |
$cflow(...)
|
cflow variable |
$r |
The result type. It is used in a cast expression. |
$w |
The wrapper type. It is used in a cast expression. |
$sig |
An array of java.lang.Class objects representing
the formal parameter types.
|
$type |
A java.lang.Class object representing
the formal result type. |
$class |
A java.lang.Class object representing
the class that declares the methodcurrently edited (the type of $0). |
Note that $_ is not available.
Javassist allows modifying only an expression included in a method body.
javassist.expr.ExprEditor is a class
for replacing an expression in a method body.
The users can define a subclass of ExprEditor
to specify how an expression is modified.
To run an ExprEditor object, the users must
call instrument() in CtMethod or
CtClass.
For example,
CtMethod cm = ... ;
cm.instrument(
new ExprEditor() {
public void edit(MethodCall m)
throws CannotCompileException
{
if (m.getClassName().equals("Point")
&& m.getMethodName().equals("move"))
m.replace("{ $1 = 0; $_ = $proceed($$); }");
}
});
searches the method body represented by cm and
replaces all calls to move() in class Point
with a block:
{ $1 = 0; $_ = $proceed($$); }
so that the first parameter to move() is always 0.
Note that the substituted code is not an expression but
a statement or a block. It cannot be or contain a try-catch statement.
The method instrument() searches a method body.
If it finds an expression such as a method call, field access, and object
creation, then it calls edit() on the given
ExprEditor object. The parameter to edit()
is an object representing the found expression. The edit()
method can inspect and replace the expression through that object.
Calling replace() on the parameter to edit()
substitutes the given statement or block for the expression. If the given
block is an empty block, that is, if replace("{}")
is executed, then the expression is removed from the method body.
If you want to insert a statement (or a block) before/after the
expression, a block like the following should be passed to
replace():
{ before-statements;
$_ = $proceed($$);
after-statements; }
whichever the expression is either a method call, field access, object creation, or others. The second statement could be:
$_ = $proceed();
if the expression is read access, or
$proceed($$);
if the expression is write access.
Local variables available in the target expression is
also available in the source text passed to replace()
if the method searched by instrument() was compiled
with the -g option (the class file includes a local variable
attribute).
A MethodCall object represents a method call.
The method replace() in
MethodCall substitutes a statement or
a block for the method call.
It receives source text representing the substitued statement or
block, in which the identifiers starting with $
have special meaning as in the source text passed to
insertBefore().
$0 |
The target object of the method call. This is not equivalent to this, which represents
the caller-side this object.$0 is null if the method is static.
|
|   | |
|   | |
$1, $2, ...     |
The parameters of the method call. |
$_
|
The resulting value of the method call. |
$r |
The result type of the method call. |
$class     |
A java.lang.Class object representing
the class declaring the method.
|
$sig     |
An array of java.lang.Class objects representing
the formal parameter types. |
$type     |
A java.lang.Class object representing
the formal result type. |
$proceed     |
The name of the method originally called in the expression. |
Here the method call means the one represented by the
MethodCall object.
The other identifiers such as $w,
$args and $$
are also available.
Unless the result type of the method call is void,
a value must be assigned to
$_ in the source text and the type of $_
is the result type.
If the result type is void, the type of $_
is Object and the value assigned to $_
is ignored.
$proceed is not a String value but special
syntax. It must be followed by an argument list surrounded by parentheses
( ).
A ConstructorCall object represents a constructor call
such as this() and super included in a constructor
body.
The method replace() in
ConstructorCall substitutes a statement or
a block for the constructor call.
It receives source text representing the substituted statement or
block, in which the identifiers starting with $
have special meaning as in the source text passed to
insertBefore().
$0 |
The target object of the constructor call.
This is equivalent to this.
|
$1, $2, ...     |
The parameters of the constructor call. |
$class     |
A java.lang.Class object representing
the class declaring the constructor.
|
$sig     |
An array of java.lang.Class objects representing
the formal parameter types. |
$proceed     |
The name of the constructor originally called in the expression. |
Here the constructor call means the one represented by the
ConstructorCall object.
The other identifiers such as $w,
$args and $$
are also available.
Since any constructor must call either a constructor of the super
class or another constructor of the same class,
the substituted statement must include a constructor call,
normally a call to $proceed().
$proceed is not a String value but special
syntax. It must be followed by an argument list surrounded by parentheses
( ).
A FieldAccess object represents field access.
The method edit() in ExprEditor
receives this object if field access is found.
The method replace() in
FieldAccess receives
source text representing the substitued statement or
block for the field access.
In the source text, the identifiers starting with $
have special meaning:
$0 |
The object containing the field accessed by the expression.
This is not equivalent to this.this represents the object that the method including the
expression is invoked on.$0 is null if the field is static.
|
|   | |
|   | |
$1 |
The value that would be stored in the field
if the expression is write access.
Otherwise, $1 is not available.
|
|   | |
$_ |
The resulting value of the field access
if the expression is read access.
Otherwise, the value stored in $_ is discarded.
|
|   | |
$r |
The type of the field if the expression is read access.
Otherwise, $r is void.
|
|   | |
$class     |
A java.lang.Class object representing
the class declaring the field.
|
$type |
A java.lang.Class object representing
the field type. |
$proceed     |
The name of a virtual method executing the original field access. . |
The other identifiers such as $w,
$args and $$
are also available.
If the expression is read access, a value must be assigned to
$_ in the source text. The type of $_
is the type of the field.
A NewExpr object represents object creation
with the new operator (not including array creation).
The method edit() in ExprEditor
receives this object if object creation is found.
The method replace() in
NewExpr receives
source text representing the substitued statement or
block for the object creation.
In the source text, the identifiers starting with $
have special meaning:
$0 |
null.
|
$1, $2, ...     |
The parameters to the constructor. |
$_ |
The resulting value of the object creation.
A newly created object must be stored in this variable. |
|   | |
$r |
The type of the created object. |
$sig     |
An array of java.lang.Class objects representing
the formal parameter types. |
$type     |
A java.lang.Class object representing
the class of the created object.
|
$proceed     |
The name of a virtual method executing the original object creation. . |
The other identifiers such as $w,
$args and $$
are also available.
A NewArray object represents array creation
with the new operator.
The method edit() in ExprEditor
receives this object if array creation is found.
The method replace() in
NewArray receives
source text representing the substitued statement or
block for the array creation.
In the source text, the identifiers starting with $
have special meaning:
$0 |
null.
|
$1, $2, ...     |
The size of each dimension. |
$_ |
The resulting value of the array creation.
A newly created array must be stored in this variable. |
|   | |
$r |
The type of the created array. |
$type     |
A java.lang.Class object representing
the class of the created array.
|
$proceed     |
The name of a virtual method executing the original array creation. . |
The other identifiers such as $w,
$args and $$
are also available.
For example, if the array creation is the following expression,
String[][] s = new String[3][4];
then the value of $1 and $2 are 3 and 4, respectively. $3 is not available.
If the array creation is the following expression,
String[][] s = new String[3][];
then the value of $1 is 3 but $2 is not available.
A Instanceof object represents an instanceof
expression.
The method edit() in ExprEditor
receives this object if an instanceof expression is found.
The method replace() in
Instanceof receives
source text representing the substitued statement or
block for the expression.
In the source text, the identifiers starting with $
have special meaning:
$0 |
null.
|
$1 |
The value on the left hand side of the original
instanceof operator.
|
$_ |
The resulting value of the expression.
The type of $_ is boolean.
|
$r |
The type on the right hand side of the instanceof operator.
|
$type |
A java.lang.Class object representing
the type on the right hand side of the instanceof operator.
|
$proceed     |
The name of a virtual method executing the original
instanceof expression.
It takes one parameter (the type is java.lang.Object)
and returns true
if the parameter value is an instance of the type on the right hand side of the original instanceof operator.
Otherwise, it returns false.
|
|   | |
|   | |
|   |
The other identifiers such as $w,
$args and $$
are also available.
A Cast object represents an expression for
explicit type casting.
The method edit() in ExprEditor
receives this object if explicit type casting is found.
The method replace() in
Cast receives
source text representing the substitued statement or
block for the expression.
In the source text, the identifiers starting with $
have special meaning:
$0 |
null.
|
$1 |
The value the type of which is explicitly cast. |
$_ |
The resulting value of the expression.
The type of $_ is the same as the type
after the explicit casting, that is, the type surrounded by ( ).
|
|   | |
$r |
the type after the explicit casting, or the type surrounded
by ( ).
|
$type |
A java.lang.Class object representing
the same type as $r.
|
$proceed     |
The name of a virtual method executing the original
type casting.
It takes one parameter of the type java.lang.Object
and returns it after
the explicit type casting specified by the original expression. |
|   | |
|   |
The other identifiers such as $w,
$args and $$
are also available.
A Handler object represents a catch
clause of try-catch statement.
The method edit() in ExprEditor
receives this object if a catch is found.
The method insertBefore() in
Handler compiles the received
source text and inserts it at the beginning of the catch clause.
In the source text, the identifiers starting with $
have meaning:
$1 |
The exception object caught by the catch clause.
|
$r |
the type of the exception caught by the catch clause.
It is used in a cast expression.
|
$w |
The wrapper type. It is used in a cast expression. |
$type     |
A java.lang.Class object representing
the type of the exception caught by the catch clause.
|
|   |
If a new exception object is assigned to $1,
it is passed to the original catch clause as the caught
exception.
Javassist allows the users to create a new method and constructor
from scratch. CtNewMethod
and CtNewConstructor provide several factory methods,
which are static methods for creating CtMethod or
CtConstructor objects.
Especially, make() creates
a CtMethod or CtConstructor object
from the given source text.
For example, this program:
CtClass point = ClassPool.getDefault().get("Point");
CtMethod m = CtNewMethod.make(
"public int xmove(int dx) { x += dx; }",
point);
point.addMethod(m);
adds a public method xmove() to class Point.
In this example, x is a int field in
the class Point.
The source text passed to make() can include the
identifiers starting with $ except $_
as in setBody().
It can also include
$proceed if the target object and the target method name
are also given to make(). For example,
CtClass point = ClassPool.getDefault().get("Point");
CtMethod m = CtNewMethod.make(
"public int ymove(int dy) { $proceed(0, dy); }",
point, "this", "move");
this program creates a method ymove() defined below:
public int ymove(int dy) { this.move(0, dy); }
Note that $proceed has been replaced with
this.move.
Javassist provides another way to add a new method. You can first create an abstract method and later give it a method body:
CtClass cc = ... ;
CtMethod m = new CtMethod(CtClass.intType, "move",
new CtClass[] { CtClass.intType }, cc);
cc.addMethod(m);
m.setBody("{ x += $1; }");
cc.setModifiers(cc.getModifiers() & ~Modifier.ABSTRACT);
Since Javassist makes a class abstract if an abstract method is
added to the class, you have to explicitly change the class back to a
non-abstract one after calling setBody().
Javassist cannot compile a method if it calls another method that
has not been added to a class. (Javassist can compile a method that
calls itself recursively.) To add mutual recursive methods to a class,
you need a trick shown below. Suppose that you want to add methods
m() and n() to a class represented
by cc:
CtClass cc = ... ;
CtMethod m = CtNewMethod.make("public abstract int m(int i);", cc);
CtMethod n = CtNewMethod.make("public abstract int n(int i);", cc);
cc.addMethod(m);
cc.addMethod(n);
m.setBody("{ return ($1 <= 0) ? 1 : (n($1 - 1) * $1); }");
n.setBody("{ return m($1); }");
cc.setModifiers(cc.getModifiers() & ~Modifier.ABSTRACT);
You must first make two abstract methods and add them to the class.
Then you can give the method bodies to these methods even if the method
bodies include method calls to each other. Finally you must change the
class to a not-abstract class since addMethod() automatically
changes a class into an abstract one if an abstract method is added.
Javassist also allows the users to create a new field.
CtClass point = ClassPool.getDefault().get("Point");
CtField f = new CtField(CtClass.intType, "z", point);
point.addField(f);
This program adds a field named z to class
Point.
If the initial value of the added field must be specified, the program shown above must be modified into:
CtClass point = ClassPool.getDefault().get("Point");
CtField f = new CtField(CtClass.intType, "z", point);
point.addField(f, "0"); // initial value is 0.
Now, the method addField() receives the second parameter,
which is the source text representing an expression computing the initial
value. This source text can be any Java expression if the result type
of the expression matches the type of the field. Note that an expression
does not end with a semi colon (;).
Furthermore, the above code can be rewritten into the following simple code:
CtClass point = ClassPool.getDefault().get("Point");
CtField f = CtField.make("public int z = 0;", point);
point.addField(f);
To remove a field or a method, call removeField()
or removeMethod() in CtClass. A
CtConstructor can be removed by removeConstructor()
in CtClass.
CtClass, CtMethod, CtField
and CtConstructor provides a convenient method
getAnnotations() for reading annotations.
It returns an annotation-type object.
For example, suppose the following annotation:
public @interface Author {
String name();
int year();
}
This annotation is used as the following:
@Author(name="Chiba", year=2005)
public class Point {
int x, y;
}
Then, the value of the annotation can be obtained by
getAnnotations().
It returns an array containing
annotation-type objects.
CtClass cc = ClassPool.getDefault().get("Point");
Object[] all = cc.getAnnotations();
Author a = (Author)all[0];
String name = a.name();
int year = a.year();
System.out.println("name: " + name + ", year: " + year);
This code snippet should print:
name: Chiba, year: 2005
Since the annoation of Point is only @Author,
the length of the array all is one
and all[0] is an Author object.
The member values of the annotation can be obtained
by calling name() and year()
on the Author object.
To use getAnnotations(), annotation types
such as Author must be included in the current
class path. They must be also accessible from a
ClassPool object. If the class file of an annotation
type is not found, Javassist cannot obtain the default values
of the members of that annotation type.
In most cases, a class modified by Javassist does not require
Javassist to run. However, some kinds of bytecode generated by the
Javassist compiler need runtime support classes, which are in the
javassist.runtime package (for details, please read
the API reference of that package). Note that the
javassist.runtime package is the only package that
classes modified by Javassist may need for running. The other
Javassist classes are never used at runtime of the modified classes.
All the class names in source code must be fully qualified
(they must include package names).
However, the java.lang package is an
exception; for example, the Javassist compiler can
resolve Object as
well as java.lang.Object.
To tell the compiler to search other packages when resolving a
class name, call importPackage() in ClassPool.
For example,
ClassPool pool = ClassPool.getDefault();
pool.importPackage("java.awt");
CtClass cc = pool.makeClass("Test");
CtField f = CtField.make("public Point p;", cc);
cc.addField(f);
The seconde line instructs the compiler
to import the java.awt package.
Thus, the third line will not throw an exception.
The compiler can recognize Point
as java.awt.Point.
Note that importPackage() does not affect
the get() method in ClassPool.
Only the compiler considers the imported packages.
The parameter to get()
must be always a fully qualified name.
In the current implementation, the Java compiler included in Javassist has several limitations with respect to the language that the compiler can accept. Those limitations are:
javassist.bytecode.annotation package
(and also getAnnotations()
in CtClass and CtBehavior).
Generics are also only partly supported. See the latter section for more details.
{ and }, are not
available unless the array dimension is one.
continue and break statements
are not supported.
For example,
class A {}
class B extends A {}
class C extends B {}
class X {
void foo(A a) { .. }
void foo(B b) { .. }
}
If the compiled expression is x.foo(new C()), where
x is an instance of X, the compiler may produce a call
to foo(A) although the compiler can correctly compile
foo((B)new C()).
# as the separator
between a class name and a static method or field name.
For example, in regular Java,
javassist.CtClass.intType.getName()
calls a method getName() on
the object indicated by the static field intType
in javassist.CtClass. In Javassist, the users can
write the expression shown above but they are recommended to
write:
javassist.CtClass#intType.getName()
so that the compiler can quickly parse the expression.