org.objectweb.asm

Class ClassWriter

java.lang.Object

org.objectweb.asm.ClassWriter

All Implemented Interfaces:

ClassVisitor

public class ClassWriter
extends Object
implements ClassVisitor

A ClassVisitor that generates classes in bytecode form. More precisely this visitor generates a byte array conforming to the Java class file format. It can be used alone, to generate a Java class "from scratch", or with one or more ClassReader and adapter class visitor to generate a modified class from one or more existing Java classes.

Author:

Eric Bruneton

Field Summary

Fields

Modifier and Type	Field and Description
static int	COMPUTE_FRAMES Flag to automatically compute the stack map frames of methods from scratch.
static int	COMPUTE_MAXS Flag to automatically compute the maximum stack size and the maximum number of local variables of methods.

Constructor Summary

Constructors

Constructor and Description
ClassWriter(ClassReader classReader, int flags) Constructs a new ClassWriter object and enables optimizations for "mostly add" bytecode transformations.
ClassWriter(int flags) Constructs a new ClassWriter object.

Method Summary

Methods

Modifier and Type	Method and Description
protected String	getCommonSuperClass(String type1, String type2) Returns the common super type of the two given types.
int	newClass(String value) Adds a class reference to the constant pool of the class being build.
int	newConst(Object cst) Adds a number or string constant to the constant pool of the class being build.
int	newField(String owner, String name, String desc) Adds a field reference to the constant pool of the class being build.
int	newMethod(String owner, String name, String desc, boolean itf) Adds a method reference to the constant pool of the class being build.
int	newNameType(String name, String desc) Adds a name and type to the constant pool of the class being build.
int	newUTF8(String value) Adds an UTF8 string to the constant pool of the class being build.
byte[]	toByteArray() Returns the bytecode of the class that was build with this class writer.
void	visit(int version, int access, String name, String signature, String superName, String[] interfaces) Visits the header of the class.
AnnotationVisitor	visitAnnotation(String desc, boolean visible) Visits an annotation of the class.
void	visitAttribute(Attribute attr) Visits a non standard attribute of the class.
void	visitEnd() Visits the end of the class.
FieldVisitor	visitField(int access, String name, String desc, String signature, Object value) Visits a field of the class.
void	visitInnerClass(String name, String outerName, String innerName, int access) Visits information about an inner class.
MethodVisitor	visitMethod(int access, String name, String desc, String signature, String[] exceptions) Visits a method of the class.
void	visitOuterClass(String owner, String name, String desc) Visits the enclosing class of the class.
void	visitSource(String file, String debug) Visits the source of the class.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

COMPUTE_MAXS

public static final int COMPUTE_MAXS

Flag to automatically compute the maximum stack size and the maximum number of local variables of methods. If this flag is set, then the arguments of the visitMaxs method of the MethodVisitor returned by the visitMethod method will be ignored, and computed automatically from the signature and the bytecode of each method.

See Also:

ClassWriter(int), Constant Field Values

COMPUTE_FRAMES

public static final int COMPUTE_FRAMES

Flag to automatically compute the stack map frames of methods from scratch. If this flag is set, then the calls to the MethodVisitor.visitFrame(int, int, java.lang.Object[], int, java.lang.Object[]) method are ignored, and the stack map frames are recomputed from the methods bytecode. The arguments of the visitMaxs method are also ignored and recomputed from the bytecode. In other words, computeFrames implies computeMaxs.

See Also:

ClassWriter(int), Constant Field Values

Constructor Detail

ClassWriter

public ClassWriter(int flags)

Constructs a new ClassWriter object.

Parameters:

flags - option flags that can be used to modify the default behavior of this class. See COMPUTE_MAXS, COMPUTE_FRAMES.

ClassWriter

public ClassWriter(ClassReader classReader,
           int flags)

Constructs a new ClassWriter object and enables optimizations for "mostly add" bytecode transformations. These optimizations are the following:

• The constant pool from the original class is copied as is in the new class, which saves time. New constant pool entries will be added at the end if necessary, but unused constant pool entries won't be removed.
• Methods that are not transformed are copied as is in the new class, directly from the original class bytecode (i.e. without emitting visit events for all the method instructions), which saves a lot of time. Untransformed methods are detected by the fact that the ClassReader receives MethodVisitor objects that come from a ClassWriter (and not from a custom ClassAdapter or any other ClassVisitor instance).

Parameters:

classReader - the ClassReader used to read the original class. It will be used to copy the entire constant pool from the original class and also to copy other fragments of original bytecode where applicable.

flags - option flags that can be used to modify the default behavior of this class. These option flags do not affect methods that are copied as is in the new class. This means that the maximum stack size nor the stack frames will be computed for these methods. See COMPUTE_MAXS, COMPUTE_FRAMES.

Method Detail

visit

public void visit(int version,
         int access,
         String name,
         String signature,
         String superName,
         String[] interfaces)

Description copied from interface: ClassVisitor

Visits the header of the class.

Specified by:

visit in interface ClassVisitor

Parameters:

version - the class version.

access - the class's access flags (see Opcodes). This parameter also indicates if the class is deprecated.

name - the internal name of the class (see getInternalName).

signature - the signature of this class. May be null if the class is not a generic one, and does not extend or implement generic classes or interfaces.

superName - the internal of name of the super class (see getInternalName). For interfaces, the super class is Object. May be null, but only for the Object class.

interfaces - the internal names of the class's interfaces (see getInternalName). May be null.

visitSource

public void visitSource(String file,
               String debug)

Description copied from interface: ClassVisitor

Visits the source of the class.

Specified by:

visitSource in interface ClassVisitor

Parameters:

file - the name of the source file from which the class was compiled. May be null.

debug - additional debug information to compute the correspondance between source and compiled elements of the class. May be null.

visitOuterClass

public void visitOuterClass(String owner,
                   String name,
                   String desc)

Description copied from interface: ClassVisitor

Visits the enclosing class of the class. This method must be called only if the class has an enclosing class.

Specified by:

visitOuterClass in interface ClassVisitor

Parameters:

owner - internal name of the enclosing class of the class.

name - the name of the method that contains the class, or null if the class is not enclosed in a method of its enclosing class.

desc - the descriptor of the method that contains the class, or null if the class is not enclosed in a method of its enclosing class.

visitAnnotation

public AnnotationVisitor visitAnnotation(String desc,
                                boolean visible)

Description copied from interface: ClassVisitor

Visits an annotation of the class.

Specified by:

visitAnnotation in interface ClassVisitor

Parameters:

desc - the class descriptor of the annotation class.

visible - true if the annotation is visible at runtime.

Returns:

a visitor to visit the annotation values, or null if this visitor is not interested in visiting this annotation.

visitAttribute

public void visitAttribute(Attribute attr)

Description copied from interface: ClassVisitor

Visits a non standard attribute of the class.

Specified by:

visitAttribute in interface ClassVisitor

Parameters:

attr - an attribute.

visitInnerClass

public void visitInnerClass(String name,
                   String outerName,
                   String innerName,
                   int access)

Description copied from interface: ClassVisitor

Visits information about an inner class. This inner class is not necessarily a member of the class being visited.

Specified by:

visitInnerClass in interface ClassVisitor

Parameters:

name - the internal name of an inner class (see getInternalName).

outerName - the internal name of the class to which the inner class belongs (see getInternalName). May be null for not member classes.

innerName - the (simple) name of the inner class inside its enclosing class. May be null for anonymous inner classes.

access - the access flags of the inner class as originally declared in the enclosing class.

visitField

public FieldVisitor visitField(int access,
                      String name,
                      String desc,
                      String signature,
                      Object value)

Description copied from interface: ClassVisitor

Visits a field of the class.

Specified by:

visitField in interface ClassVisitor

Parameters:

access - the field's access flags (see Opcodes). This parameter also indicates if the field is synthetic and/or deprecated.

name - the field's name.

desc - the field's descriptor (see Type).

signature - the field's signature. May be null if the field's type does not use generic types.

value - the field's initial value. This parameter, which may be null if the field does not have an initial value, must be an Integer, a Float, a Long, a Double or a String (for int, float, long or String fields respectively). This parameter is only used for static fields. Its value is ignored for non static fields, which must be initialized through bytecode instructions in constructors or methods.

Returns:

a visitor to visit field annotations and attributes, or null if this class visitor is not interested in visiting these annotations and attributes.

visitMethod

public MethodVisitor visitMethod(int access,
                        String name,
                        String desc,
                        String signature,
                        String[] exceptions)

Description copied from interface: ClassVisitor

Visits a method of the class. This method must return a new MethodVisitor instance (or null) each time it is called, i.e., it should not return a previously returned visitor.

Specified by:

visitMethod in interface ClassVisitor

Parameters:

access - the method's access flags (see Opcodes). This parameter also indicates if the method is synthetic and/or deprecated.

name - the method's name.

desc - the method's descriptor (see Type).

signature - the method's signature. May be null if the method parameters, return type and exceptions do not use generic types.

exceptions - the internal names of the method's exception classes (see getInternalName). May be null.

Returns:

an object to visit the byte code of the method, or null if this class visitor is not interested in visiting the code of this method.

visitEnd

public void visitEnd()

Description copied from interface: ClassVisitor

Visits the end of the class. This method, which is the last one to be called, is used to inform the visitor that all the fields and methods of the class have been visited.

Specified by:

visitEnd in interface ClassVisitor

toByteArray

public byte[] toByteArray()

Returns the bytecode of the class that was build with this class writer.

Returns:

the bytecode of the class that was build with this class writer.

newConst

public int newConst(Object cst)

Adds a number or string constant to the constant pool of the class being build. Does nothing if the constant pool already contains a similar item. This method is intended for Attribute sub classes, and is normally not needed by class generators or adapters.

Parameters:

cst - the value of the constant to be added to the constant pool. This parameter must be an Integer, a Float, a Long, a Double or a String.

Returns:

the index of a new or already existing constant item with the given value.

newUTF8

public int newUTF8(String value)

Adds an UTF8 string to the constant pool of the class being build. Does nothing if the constant pool already contains a similar item. This method is intended for Attribute sub classes, and is normally not needed by class generators or adapters.

Parameters:

value - the String value.

Returns:

the index of a new or already existing UTF8 item.

newClass

public int newClass(String value)

Adds a class reference to the constant pool of the class being build. Does nothing if the constant pool already contains a similar item. This method is intended for Attribute sub classes, and is normally not needed by class generators or adapters.

Parameters:

value - the internal name of the class.

Returns:

the index of a new or already existing class reference item.

newField

public int newField(String owner,
           String name,
           String desc)

Adds a field reference to the constant pool of the class being build. Does nothing if the constant pool already contains a similar item. This method is intended for Attribute sub classes, and is normally not needed by class generators or adapters.

Parameters:

owner - the internal name of the field's owner class.

name - the field's name.

desc - the field's descriptor.

Returns:

the index of a new or already existing field reference item.

newMethod

public int newMethod(String owner,
            String name,
            String desc,
            boolean itf)

Adds a method reference to the constant pool of the class being build. Does nothing if the constant pool already contains a similar item. This method is intended for Attribute sub classes, and is normally not needed by class generators or adapters.

Parameters:

owner - the internal name of the method's owner class.

name - the method's name.

desc - the method's descriptor.

itf - true if owner is an interface.

Returns:

the index of a new or already existing method reference item.

newNameType

public int newNameType(String name,
              String desc)

Adds a name and type to the constant pool of the class being build. Does nothing if the constant pool already contains a similar item. This method is intended for Attribute sub classes, and is normally not needed by class generators or adapters.

Parameters:

name - a name.

desc - a type descriptor.

Returns:

the index of a new or already existing name and type item.

getCommonSuperClass

protected String getCommonSuperClass(String type1,
                         String type2)

Returns the common super type of the two given types. The default implementation of this method loads the two given classes and uses the java.lang.Class methods to find the common super class. It can be overridden to compute this common super type in other ways, in particular without actually loading any class, or to take into account the class that is currently being generated by this ClassWriter, which can of course not be loaded since it is under construction.

Parameters:

type1 - the internal name of a class.

type2 - the internal name of another class.

Returns:

the internal name of the common super class of the two given classes.