Javassist Tutorial

1. Reading and writing bytecode

The class Javassist.CtClass is an abstract representation of a class file. A CtClass (compile-time class) object is a handle for dealing with a class file.

ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("test.Rectangle");
cc.setSuperclass(pool.get("test.Point"));
cc.writeFile();

Defining a new class

To define a new class from scratch, makeClass() must be called on a ClassPool.

ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.makeClass("Point");

Frozen classes

If a CtClass object is converted into a class file by writeFile(), toClass(), or toBytecode(), Javassist freezes that CtClass object. Further modifications of that CtClass object are not permitted. This is for warning the developers when they attempt to modify a class file that has been already loaded since the JVM does not allow reloading a class.

To disallow pruning a particular CtClass, stopPruning() must be called on that object in advance

Class search path

ClassPool pool = ClassPool.getDefault();
pool.insertClassPath("/usr/local/javalib");

2. ClassPool

A ClassPool object is a container of CtClass objects. Once a CtClass object is created, it is recorded in a ClassPool for ever. This is because a compiler may need to access the CtClass object later when it compiles source code that refers to the class represented by that CtClass.

Avoid out of memory

CtClass cc = ... ;
cc.writeFile();
cc.detach();

ClassPool cp = new ClassPool(true);
// if needed, append an extra search path by appendClassPath()
ClassPool cp = new ClassPool();
cp.appendSystemPath();  // or append another path by appendClassPath()

Cascaded ClassPools

Multiple ClassPool objects can be cascaded like java.lang.ClassLoader.

ClassPool parent = ClassPool.getDefault();
ClassPool child = new ClassPool(parent);
child.insertClassPath("./classes");

ClassPool parent = ClassPool.getDefault();
ClassPool child = new ClassPool(parent);
child.appendSystemPath();         // the same class path as the default one.
child.childFirstLookup = true;    // changes the behavior of the child.

Changing a class name for defining a new class

ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("Point");
cc.setName("Pair");

ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("Point");
CtClass cc1 = pool.get("Point");   // cc1 is identical to cc.
cc.setName("Pair");
CtClass cc2 = pool.get("Pair");    // cc2 is identical to cc.
CtClass cc3 = pool.get("Point");   // cc3 is not identical to cc.

Renaming a frozen class for defining a new class

Once a CtClass object is converted into a class file by writeFile() or toBytecode(), Javassist rejects further modifications of that CtClass object.

ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("Point");
cc.writeFile();
CtClass cc2 = pool.getAndRename("Point", "Pair");

If getAndRename() is called, the ClassPool first reads Point.class for creating a new CtClass object representing Point class. However, it renames that CtClass object from Point to Pair before it records that CtClass object in a hash table. Thus getAndRename() can be executed after writeFile() or toBytecode() is called on the the CtClass object representing Point class.

3. Class loader

1. Get a CtClass object by calling ClassPool.get(),

2. Modify it, and

3. Call writeFile() or toBytecode() on that CtClass object to obtain a modified class file.

3.1 The toClass method in CtClass

The CtClass provides a convenience method toClass(), which requests the context class loader for the current thread to load the class represented by the CtClass object. To call this method, the caller must have appropriate permission; otherwise, a SecurityException may be thrown.

public class Hello {
    public void say() {
        System.out.println("Hello");
    }
}

public class Test {
    public static void main(String[] args) throws Exception {
        ClassPool cp = ClassPool.getDefault();
        CtClass cc = cp.get("Hello");
        CtMethod m = cc.getDeclaredMethod("say");
        m.insertBefore("{ System.out.println(\"Hello.say():\"); }");
        Class c = cc.toClass();
        Hello h = (Hello)c.newInstance();
        h.say();
    }
}

3.2 Class loading in Java

In Java, multiple class loaders can coexist and each class loader creates its own name space. Different class loaders can load different class files with the same class name. The loaded two classes are regarded as different ones. This feature enables us to run multiple application programs on a single JVM even if these programs include different classes with the same name.

Note: The JVM does not allow dynamically reloading a class. Once a class loader loads a class, it cannot reload a modified version of that class during runtime. Thus, you cannot alter the definition of a class after the JVM loads it. However, the JPDA (Java Platform Debugger Architecture) provides limited ability for reloading a class.

Dynamic Class Loading Example

ClassLoader working mechanism:

1. A ClassLoader instance checks if the class was already loaded.

2. If not loaded, it delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself.

3. If parent class loader cannot load class, it attempt to load the class or resource by itself.

3.3 Using javassist.Loader

Javassist provides a class loader javassist.Loader. This class loader uses a javassist.ClassPool object for reading a class file.

public class MyTranslator implements Translator {
    void start(ClassPool pool)
        throws NotFoundException, CannotCompileException {}
    void onLoad(ClassPool pool, String classname)
        throws NotFoundException, CannotCompileException
    {
        CtClass cc = pool.get(classname);
        cc.setModifiers(Modifier.PUBLIC);
    }
}

If the users want to modify a class on demand when it is loaded, the users can add an event listener to a javassist.Loader.

The event-listener class must implement javassist.Translator:

• The method start() is called when this event listener is added to a javassist.Loader object by addTranslator() in javassist.Loader.

• The method onLoad() is called before javassist.Loader loads a class. onLoad() can modify the definition of the loaded class.

Note that onLoad() does not have to call toBytecode() or writeFile() since javassist.Loader calls these methods to obtain a class file.

public class Main2 {
  public static void main(String[] args) throws Throwable {
     Translator t = new MyTranslator();
     ClassPool pool = ClassPool.getDefault();
     Loader cl = new Loader();
     cl.addTranslator(pool, t);
     cl.run("MyApp", args);
  }
}

javassist.Loader searches for classes in a different order from java.lang.ClassLoader. ClassLoader first delegates the loading operations to the parent class loader and then attempts to load the classes only if the parent class loader cannot find them. On the other hand, javassist.Loader attempts to load the classes before delegating to the parent class loader. It delegates only if:

• the classes are not found by calling get() on a ClassPool object, or

• the classes have been specified by using delegateLoadingOf() to be loaded by the parent class loader.

3.4 Writing a class loader

public class SampleLoader extends ClassLoader {
    /* Call MyApp.main().
     */
    public static void main(String[] args) throws Throwable {
        SampleLoader s = new SampleLoader();
        Class c = s.loadClass("MyApp");
        c.getDeclaredMethod("main", new Class[] { String[].class })
         .invoke(null, new Object[] { args });
    }

    private ClassPool pool;

    public SampleLoader() throws NotFoundException {
        pool = new ClassPool();
        pool.insertClassPath("./class"); // MyApp.class must be there.
    }

    /* Finds a specified class.
     * The bytecode for that class can be modified.
     */
    protected Class findClass(String name) throws ClassNotFoundException {
        try {
            CtClass cc = pool.get(name);
            // modify the CtClass object here
            byte[] b = cc.toBytecode();
            return defineClass(name, b, 0, b.length);
        } catch (NotFoundException e) {
            throw new ClassNotFoundException();
        } catch (IOException e) {
            throw new ClassNotFoundException();
        } catch (CannotCompileException e) {
            throw new ClassNotFoundException();
        }
    }
}

3.5 Modifying a system class

The system classes like java.lang.String cannot be loaded by a class loader other than the system class loader.

the system classes must be statically modified.

ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("java.lang.String");
CtField f = new CtField(CtClass.intType, "hiddenValue", cc);
f.setModifiers(Modifier.PUBLIC);
cc.addField(f);
cc.writeFile(".");

java -Xbootclasspath/p:. MyApp arg1 arg2...

Note: Applications that use this technique for the purpose of overriding a system class in rt.jar should not be deployed as doing so would contravene the Java 2 Runtime Environment binary code license.

3.6 Reloading a class at runtime

If the JVM is launched with the JPDA (Java Platform Debugger Architecture) enabled, a class is dynamically reloadable. After the JVM loads a class, the old version of the class definition can be unloaded and a new one can be reloaded again. That is, the definition of that class can be dynamically modified during runtime. However, the new class definition must be somewhat compatible to the old one. The JVM does not allow schema changes between the two versions. They have the same set of methods and fields.

Javassist provides a convenient class for reloading a class at runtime. For more information, see the API documentation of javassist.tools.HotSwapper.

4. Introspection and customization

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 to add a new field, constructor, and method. Instrumenting a method body is also possible.

4.1 Inserting source text at the beginning/end of a method body

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:

identifiers	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.

$0, $1, $2, …

$1 represents the first parameter, $2 represents the second parameter, and so on.

$0 is equivalent to this. If the method is static, $0 is not available.

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();

$$

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)

$cflow

To use $cflow, first declare that $cflow is used for monitoring calls to the method fact():

CtMethod cm = ...;
cm.useCflow("fact");

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);");

$r

$r represents the result type (return type) of the method.

• 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

• Even if the result type is void, the following return statement is valid: return ($r)result;. This return statement is regarded as the equivalent of the return statement without a resulting value: return;

$w

If the type of the expression following ($w) is not a primitive type, then ($w) does nothing.

$_

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.

Note that the value of $_ is never thrown to the caller; it is rather discarded.

addCatch()

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.

CtClass etype = ClassPool.getDefault().get("java.io.IOException");
m.addCatch("{ System.out.println($e); throw $e; }", etype);

Note that the inserted code fragment must end with a throw or return statement.

4.2 Altering a method body

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.

Note that $_ is not available.

Substituting source text for an existing expression

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.

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($$); }");
        }
    });

Note that the substituted code is not an expression but a statement or a block. It cannot be or contain a try-catch statement.

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; }

javassist.expr.MethodCall

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().

javassist.expr.ConstructorCall

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().

javassist.expr.FieldAccess

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.

javassist.expr.NewExpr

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.

javassist.expr.NewArray

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.

javassist.expr.Instanceof

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.

javassist.expr.Cast

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.

javassist.expr.Handler

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.

4.3 Adding a new method or field

Adding a method

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.

CtClass point = ClassPool.getDefault().get("Point");
CtMethod m = CtNewMethod.make(
                 "public int xmove(int dx) { x += dx; }",
                 point);
point.addMethod(m);

CtClass point = ClassPool.getDefault().get("Point");
CtMethod m = CtNewMethod.make(
                 "public int ymove(int dy) { $proceed(0, dy); }",
                 point, "this", "move");

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().

Mutual recursive methods

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.

Adding a field

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);
point.addField(f, "0"); // initial value is 0

CtClass point = ClassPool.getDefault().get("Point");
CtField f = CtField.make("public int z = 0;", point);
point.addField(f);

Removing a member

To remove a field or a method, call removeField() or removeMethod() in CtClass. A CtConstructor can be removed by removeConstructor() in CtClass.

4.4 Annotations

CtClass, CtMethod, CtField and CtConstructor provides a convenient method getAnnotations() for reading annotations. It returns an annotation-type object.

public @interface Author {
    String name();
    int year();
}

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.

4.5 Runtime support classes

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.

4.6 Import

All the class names in source code must be fully qualified (they must include package names). However, the java.lang package is an exception

ClassPool pool = ClassPool.getDefault();
pool.importPackage("java.awt");
CtClass cc = pool.makeClass("Test");
CtField f = CtField.make("public Point p;", cc);
cc.addField(f);

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.

4.7 Limitations

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:

• The new syntax introduced by J2SE 5.0 (including enums and generics) has not been supported. Annotations are supported by the low level API of Javassist. See the javassist.bytecode.annotation package (and also getAnnotations() in CtClass and CtBehavior). Generics are also only partly supported.

• Array initializers, a comma-separated list of expressions enclosed by braces { and }, are not available unless the array dimension is one.

• Inner classes or anonymous classes are not supported. Note that this is a limitation of the compiler only. It cannot compile source code including an anonymous-class declaration. Javassist can read and modify a class file of inner/anonymous class.

• Labeled continue and break statements are not supported.

• The compiler does not correctly implement the Java method dispatch algorithm. The compiler may confuse if methods defined in a class have the same name but take different parameter lists.

• The users are recommended to use # as the separator between a class name and a static method or field name.

5. Bytecode level API

Javassist also provides lower-level API for directly editing a class file.

If you want to just produce a simple class file, javassist.bytecode.ClassFileWriter might provide the best API for you. It is much faster than javassist.bytecode.ClassFile although its API is minimum.

5.1 Obtaining a ClassFile object

BufferedInputStream fin = new BufferedInputStream(new FileInputStream("Point.class"));
ClassFile cf = new ClassFile(new DataInputStream(fin));

You can create a new class file from scratch.

ClassFile cf = new ClassFile(false, "test.Foo", null);
cf.setInterfaces(new String[] { "java.lang.Cloneable" });
FieldInfo f = new FieldInfo(cf.getConstPool(), "width", "I");
f.setAccessFlags(AccessFlag.PUBLIC);
cf.addField(f);
cf.write(new DataOutputStream(new FileOutputStream("Foo.class")));

5.2 Adding and removing a member

ClassFile provides addField() and addMethod() for adding a field or a method (note that a constructor is regarded as a method at the bytecode level). It also provides addAttribute() for adding an attribute to the class file.

Note that FieldInfo, MethodInfo, and AttributeInfo objects include a link to a ConstPool (constant pool table) object. The ConstPool object must be common to the ClassFile object and a FieldInfo (or MethodInfo etc.) object that is added to that ClassFile object. In other words, a FieldInfo (or MethodInfo etc.) object must not be shared among different ClassFile objects.

To remove a field or a method from a ClassFile object, you must first obtain a java.util.List object containing all the fields of the class. getFields() and getMethods() return the lists. A field or a method can be removed by calling remove() on the List object. An attribute can be removed in a similar way. Call getAttributes() in FieldInfo or MethodInfo to obtain the list of attributes, and remove one from the list.

5.3 Traversing a method body

To examine every bytecode instruction in a method body, CodeIterator is useful.

ClassFile cf = ... ;
MethodInfo minfo = cf.getMethod("move");    // we assume move is not overloaded.
CodeAttribute ca = minfo.getCodeAttribute();
CodeIterator ci = ca.iterator();
while (ci.hasNext()) {
    int index = ci.next();
    int op = ci.byteAt(index);
    System.out.println(Mnemonic.OPCODE[op]);
}

A CodeIterator object allows you to visit every bytecode instruction one by one from the beginning to the end. The following methods are part of the methods declared in CodeIterator:

• void begin(): Move to the first instruction.

• void move(int index): Move to the instruction specified by the given index.

• boolean hasNext(): Returns true if there is more instructions.

• int next(): Returns the index of the next instruction. Note that it does not return the opcode of the next instruction.

• int byteAt(int index): Returns the unsigned 8bit value at the index.

• int u16bitAt(int index): Returns the unsigned 16bit value at the index.

• int write(byte[] code, int index): Writes a byte array at the index.

• void insert(int index, byte[] code): Inserts a byte array at the index. Branch offsets etc. are automatically adjusted.

5.4 Producing a bytecode sequence

A Bytecode object represents a sequence of bytecode instructions. It is a growable array of bytecode.

ConstPool cp = ...;    // constant pool table
Bytecode b = new Bytecode(cp, 1, 0);
b.addIconst(3);
b.addReturn(CtClass.intType);
CodeAttribute ca = b.toCodeAttribute();

Bytecode can be used to construct a method.

ClassFile cf = ...
Bytecode code = new Bytecode(cf.getConstPool());
code.addAload(0);
code.addInvokespecial("java/lang/Object", MethodInfo.nameInit, "()V");
code.addReturn(null);
code.setMaxLocals(1);
MethodInfo minfo = new MethodInfo(cf.getConstPool(), MethodInfo.nameInit, "()V");
minfo.setCodeAttribute(code.toCodeAttribute());
cf.addMethod(minfo);

5.5 Annotations (Meta tags)

Annotations are stored in a class file as runtime invisible (or visible) annotations attribute. These attributes can be obtained from ClassFile, MethodInfo, or FieldInfo objects. Call getAttribute(AnnotationsAttribute.invisibleTag) on those objects. For more details, see the javadoc manual of javassist.bytecode.AnnotationsAttribute class and the javassist.bytecode.annotation package.

Javassist also let you access annotations by the higher-level API. If you want to access annotations through CtClass, call getAnnotations() in CtClass or CtBehavior.

6. Generics

The lower-level API of Javassist fully supports generics introduced by Java 5. On the other hand, the higher-level API such as CtClass does not directly support generics. However, this is not a serious problem for bytecode transformation.

The generics of Java is implemented by the erasure technique. After compilation, all type parameters are dropped off.

Note that no type parameters are necessary. Since get returns an Object, an explicit type cast is needed at the caller site if the source code is compiled by Javassist. For example, if the type parameter T is String, then (String) must be inserted as follows:

Wrapper w = ...
String s = (String)w.get();

7. Varargs

Currently, Javassist does not directly support varargs. So to make a method with varargs, you must explicitly set a method modifier.

CtClass cc = /* target class */;
CtMethod m = CtMethod.make("public int length(int[] args) { return args.length; }", cc);
m.setModifiers(m.getModifiers() | Modifier.VARARGS);
cc.addMethod(m);

8. J2ME

If you modify a class file for the J2ME execution environment, you must perform preverification. Preverifying is basically producing stack maps, which is similar to stack map tables introduced into J2SE at JDK 1.6. Javassist maintains the stack maps for J2ME only if javassist.bytecode.MethodInfo.doPreverify is true.

You can also manually produce a stack map for a modified method. For a given method represented by a CtMethod object m, you can produce a stack map by calling the following methods:

m.getMethodInfo().rebuildStackMapForME(cpool);

Here, cpool is a ClassPool object, which is available by calling getClassPool() on a CtClass object. A ClassPool object is responsible for finding class files from given class pathes. To obtain all the CtMethod objects, call the getDeclaredMethods method on a CtClass object.

9. Boxing/Unboxing

Boxing and unboxing in Java are syntactic sugar. There is no bytecode for boxing or unboxing. So the compiler of Javassist does not support them.

For Javassist, however, you must explicitly convert a value type from int to Integer:

Integer i = new Integer(3);

10. Debug

Set CtClass.debugDump to a directory name. Then all class files modified and generated by Javassist are saved in that directory. To stop this, set CtClass.debugDump to null. The default value is null.

CtClass.debugDump = "./dump";

All modified class files are saved in ./dump.

Results

• ReadWriteBytecodeTest

• ClassPoolTest

• ClassLoaderTest

• IntrospectionAndCustomizationTest

• BytecodeLevelApiTest

What Is the Bytecode?

cd java-repositories/jdk8/t5750/javassist/domain
javap -c Pair.class

public class t5750.javassist.domain.Pair extends java.awt.geom.Point2D implements java.io.Serializable {
  public t5750.javassist.domain.Pair(int, int);
    Code:
       0: aload_0
       1: invokespecial #80                 // Method java/awt/geom/Point2D."<init>":()V
       4: aload_0
       5: iload_1
       6: putfield      #75                 // Field x:I
       9: aload_0
      10: iload_2
      11: putfield      #76                 // Field y:I
      14: return

  public void move(int, int);
    Code:
       0: aload_0
       1: iload_1
       2: putfield      #75                 // Field x:I
       5: aload_0
       6: iload_2
       7: putfield      #76                 // Field y:I
      10: return
}

Let’s analyze the bytecode instructions of the move() method:

• aload_0 instruction is loading a reference onto the stack from the local variable 0

• iload_1 is loading an int value from the local variable 1

• putfield is setting a field x of our object. All operations are analogical for field y

• The last instruction is a return

References

• Getting Started with Javassist

• Dynamic Class Loading Example

• Introduction to Javassist