org.objectweb.asm

Class ClassWriter

java.lang.Object

org.objectweb.asm.ClassVisitor

org.objectweb.asm.ClassWriter

public class ClassWriter
extends ClassVisitor

A ClassVisitor that generates a corresponding ClassFile structure, as defined in the Java Virtual Machine Specification (JVMS). It can be used alone, to generate a Java class "from scratch", or with one or more ClassReader and adapter ClassVisitor to generate a modified class from one or more existing Java classes.

Author:

Eric Bruneton

See Also:

JVMS 4

Field Summary

Fields

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

Fields inherited from class org.objectweb.asm.ClassVisitor

api, cv

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

Modifier and Type	Method and Description
protected ClassLoader	getClassLoader() Returns the ClassLoader to be used by the default implementation of getCommonSuperClass(String, String), that of this ClassWriter's runtime type by default.
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 value) Adds a number or string constant to the constant pool of the class being build.
int	newConstantDynamic(String name, String descriptor, Handle bootstrapMethodHandle, Object... bootstrapMethodArguments) Adds a dynamic constant reference to the constant pool of the class being build.
int	newField(String owner, String name, String descriptor) Adds a field reference to the constant pool of the class being build.
int	newHandle(int tag, String owner, String name, String descriptor) Deprecated. this method is superseded by newHandle(int, String, String, String, boolean).
int	newHandle(int tag, String owner, String name, String descriptor, boolean isInterface) Adds a handle to the constant pool of the class being build.
int	newInvokeDynamic(String name, String descriptor, Handle bootstrapMethodHandle, Object... bootstrapMethodArguments) Adds an invokedynamic reference to the constant pool of the class being build.
int	newMethod(String owner, String name, String descriptor, boolean isInterface) Adds a method reference to the constant pool of the class being build.
int	newMethodType(String methodDescriptor) Adds a method type reference to the constant pool of the class being build.
int	newModule(String moduleName) Adds a module reference to the constant pool of the class being build.
int	newNameType(String name, String descriptor) Adds a name and type to the constant pool of the class being build.
int	newPackage(String packageName) Adds a package reference 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 content of the class file that was built by this ClassWriter.
void	visit(int version, int access, String name, String signature, String superName, String[] interfaces) Visits the header of the class.
AnnotationVisitor	visitAnnotation(String descriptor, boolean visible) Visits an annotation of the class.
void	visitAttribute(Attribute attribute) Visits a non standard attribute of the class.
void	visitEnd() Visits the end of the class.
FieldVisitor	visitField(int access, String name, String descriptor, 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 descriptor, String signature, String[] exceptions) Visits a method of the class.
ModuleVisitor	visitModule(String name, int access, String version) Visit the module corresponding to the class.
void	visitNestHost(String nestHost) Visits the nest host class of the class.
void	visitNestMember(String nestMember) Visits a member of the nest.
void	visitOuterClass(String owner, String name, String descriptor) Visits the enclosing class of the class.
void	visitPermittedSubtypeExperimental(String permittedSubtype) Deprecated. this API is experimental.
RecordComponentVisitor	visitRecordComponent(String name, String descriptor, String signature) Visits a record component of the class.
void	visitSource(String file, String debug) Visits the source of the class.
AnnotationVisitor	visitTypeAnnotation(int typeRef, TypePath typePath, String descriptor, boolean visible) Visits an annotation on a type in the class signature.

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

A 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 MethodVisitor.visitMaxs(int, int) method of the MethodVisitor returned by the visitMethod(int, java.lang.String, java.lang.String, java.lang.String, java.lang.String[]) method will be ignored, and computed automatically from the signature and the bytecode of each method.

Note: for classes whose version is Opcodes.V1_7 of more, this option requires valid stack map frames. The maximum stack size is then computed from these frames, and from the bytecode instructions in between. If stack map frames are not present or must be recomputed, used COMPUTE_FRAMES instead.

See Also:

ClassWriter(int), Constant Field Values

COMPUTE_FRAMES

public static final int COMPUTE_FRAMES

A 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 MethodVisitor.visitMaxs(int, int) method are also ignored and recomputed from the bytecode. In other words, COMPUTE_FRAMES implies COMPUTE_MAXS.

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. Must be zero or more of COMPUTE_MAXS and 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 and bootstrap methods from the original class are copied as is in the new class, which saves time. New constant pool entries and new bootstrap methods will be added at the end if necessary, but unused constant pool entries or bootstrap methods 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 any other ClassVisitor instance).

Parameters:

classReader - the ClassReader used to read the original class. It will be used to copy the entire constant pool and bootstrap methods 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.Must be zero or more of COMPUTE_MAXS and COMPUTE_FRAMES. These option flags do not affect methods that are copied as is in the new class. This means that neither the maximum stack size nor the stack frames will be computed for these methods.

Method Detail

visit

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

Description copied from class: ClassVisitor

Visits the header of the class.

Overrides:

visit in class ClassVisitor

Parameters:

version - the class version. The minor version is stored in the 16 most significant bits, and the major version in the 16 least significant bits.

access - the class's access flags (see Opcodes). This parameter also indicates if the class is deprecated Opcodes.ACC_DEPRECATED or a record Opcodes.ACC_RECORD.

name - the internal name of the class (see Type.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 Type.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 Type.getInternalName()). May be null.

visitSource

public final void visitSource(String file,
                              String debug)

Description copied from class: ClassVisitor

Visits the source of the class.

Overrides:

visitSource in class 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 correspondence between source and compiled elements of the class. May be null.

visitModule

public final ModuleVisitor visitModule(String name,
                                       int access,
                                       String version)

Description copied from class: ClassVisitor

Visit the module corresponding to the class.

Overrides:

visitModule in class ClassVisitor

Parameters:

name - the fully qualified name (using dots) of the module.

access - the module access flags, among ACC_OPEN, ACC_SYNTHETIC and ACC_MANDATED.

version - the module version, or null.

Returns:

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

visitNestHost

public final void visitNestHost(String nestHost)

Description copied from class: ClassVisitor

Visits the nest host class of the class. A nest is a set of classes of the same package that share access to their private members. One of these classes, called the host, lists the other members of the nest, which in turn should link to the host of their nest. This method must be called only once and only if the visited class is a non-host member of a nest. A class is implicitly its own nest, so it's invalid to call this method with the visited class name as argument.

Overrides:

visitNestHost in class ClassVisitor

Parameters:

nestHost - the internal name of the host class of the nest.

visitOuterClass

public final void visitOuterClass(String owner,
                                  String name,
                                  String descriptor)

Description copied from class: ClassVisitor

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

Overrides:

visitOuterClass in class 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.

descriptor - 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 final AnnotationVisitor visitAnnotation(String descriptor,
                                               boolean visible)

Description copied from class: ClassVisitor

Visits an annotation of the class.

Overrides:

visitAnnotation in class ClassVisitor

Parameters:

descriptor - 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.

visitTypeAnnotation

public final AnnotationVisitor visitTypeAnnotation(int typeRef,
                                                   TypePath typePath,
                                                   String descriptor,
                                                   boolean visible)

Description copied from class: ClassVisitor

Visits an annotation on a type in the class signature.

Overrides:

visitTypeAnnotation in class ClassVisitor

Parameters:

typeRef - a reference to the annotated type. The sort of this type reference must be TypeReference.CLASS_TYPE_PARAMETER, TypeReference.CLASS_TYPE_PARAMETER_BOUND or TypeReference.CLASS_EXTENDS. See TypeReference.

typePath - the path to the annotated type argument, wildcard bound, array element type, or static inner type within 'typeRef'. May be null if the annotation targets 'typeRef' as a whole.

descriptor - 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 final void visitAttribute(Attribute attribute)

Description copied from class: ClassVisitor

Visits a non standard attribute of the class.

Overrides:

visitAttribute in class ClassVisitor

Parameters:

attribute - an attribute.

visitNestMember

public final void visitNestMember(String nestMember)

Description copied from class: ClassVisitor

Visits a member of the nest. A nest is a set of classes of the same package that share access to their private members. One of these classes, called the host, lists the other members of the nest, which in turn should link to the host of their nest. This method must be called only if the visited class is the host of a nest. A nest host is implicitly a member of its own nest, so it's invalid to call this method with the visited class name as argument.

Overrides:

visitNestMember in class ClassVisitor

Parameters:

nestMember - the internal name of a nest member.

visitPermittedSubtypeExperimental

@Deprecated
public final void visitPermittedSubtypeExperimental(String permittedSubtype)

Deprecated. this API is experimental.

Experimental, use at your own risk.

Overrides:

visitPermittedSubtypeExperimental in class ClassVisitor

Parameters:

permittedSubtype - the internal name of a permitted subtype.

visitInnerClass

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

Description copied from class: ClassVisitor

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

Overrides:

visitInnerClass in class ClassVisitor

Parameters:

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

outerName - the internal name of the class to which the inner class belongs (see Type.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.

visitRecordComponent

public final RecordComponentVisitor visitRecordComponent(String name,
                                                         String descriptor,
                                                         String signature)

Description copied from class: ClassVisitor

Visits a record component of the class.

Overrides:

visitRecordComponent in class ClassVisitor

Parameters:

name - the record component name.

descriptor - the record component descriptor (see Type).

signature - the record component signature. May be null if the record component type does not use generic types.

Returns:

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

visitField

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

Description copied from class: ClassVisitor

Visits a field of the class.

Overrides:

visitField in class 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.

descriptor - 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 final MethodVisitor visitMethod(int access,
                                       String name,
                                       String descriptor,
                                       String signature,
                                       String[] exceptions)

Description copied from class: 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.

Overrides:

visitMethod in class 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.

descriptor - 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 Type.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 final void visitEnd()

Description copied from class: 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.

Overrides:

visitEnd in class ClassVisitor

toByteArray

public byte[] toByteArray()

Returns the content of the class file that was built by this ClassWriter.

Returns:

the binary content of the JVMS ClassFile structure that was built by this ClassWriter.

Throws:

ClassTooLargeException - if the constant pool of the class is too large.

MethodTooLargeException - if the Code attribute of a method is too large.

newConst

public int newConst(Object value)

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:

value - 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.

newMethodType

public int newMethodType(String methodDescriptor)

Adds a method type 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:

methodDescriptor - method descriptor of the method type.

Returns:

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

newModule

public int newModule(String moduleName)

Adds a module 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:

moduleName - name of the module.

Returns:

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

newPackage

public int newPackage(String packageName)

Adds a package 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:

packageName - name of the package in its internal form.

Returns:

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

newHandle

@Deprecated
public int newHandle(int tag,
                                 String owner,
                                 String name,
                                 String descriptor)

Deprecated. this method is superseded by newHandle(int, String, String, String, boolean).

Adds a handle 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:

tag - the kind of this handle. Must be Opcodes.H_GETFIELD, Opcodes.H_GETSTATIC, Opcodes.H_PUTFIELD, Opcodes.H_PUTSTATIC, Opcodes.H_INVOKEVIRTUAL, Opcodes.H_INVOKESTATIC, Opcodes.H_INVOKESPECIAL, Opcodes.H_NEWINVOKESPECIAL or Opcodes.H_INVOKEINTERFACE.

owner - the internal name of the field or method owner class.

name - the name of the field or method.

descriptor - the descriptor of the field or method.

Returns:

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

newHandle

public int newHandle(int tag,
                     String owner,
                     String name,
                     String descriptor,
                     boolean isInterface)

Adds a handle 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:

tag - the kind of this handle. Must be Opcodes.H_GETFIELD, Opcodes.H_GETSTATIC, Opcodes.H_PUTFIELD, Opcodes.H_PUTSTATIC, Opcodes.H_INVOKEVIRTUAL, Opcodes.H_INVOKESTATIC, Opcodes.H_INVOKESPECIAL, Opcodes.H_NEWINVOKESPECIAL or Opcodes.H_INVOKEINTERFACE.

owner - the internal name of the field or method owner class.

name - the name of the field or method.

descriptor - the descriptor of the field or method.

isInterface - true if the owner is an interface.

Returns:

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

newConstantDynamic

public int newConstantDynamic(String name,
                              String descriptor,
                              Handle bootstrapMethodHandle,
                              Object... bootstrapMethodArguments)

Adds a dynamic constant 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:

name - name of the invoked method.

descriptor - field descriptor of the constant type.

bootstrapMethodHandle - the bootstrap method.

bootstrapMethodArguments - the bootstrap method constant arguments.

Returns:

the index of a new or already existing dynamic constant reference item.

newInvokeDynamic

public int newInvokeDynamic(String name,
                            String descriptor,
                            Handle bootstrapMethodHandle,
                            Object... bootstrapMethodArguments)

Adds an invokedynamic 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:

name - name of the invoked method.

descriptor - descriptor of the invoke method.

bootstrapMethodHandle - the bootstrap method.

bootstrapMethodArguments - the bootstrap method constant arguments.

Returns:

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

newField

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

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.

descriptor - 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 descriptor,
                     boolean isInterface)

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.

descriptor - the method's descriptor.

isInterface - 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 descriptor)

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.

descriptor - 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.

getClassLoader

protected ClassLoader getClassLoader()

Returns the ClassLoader to be used by the default implementation of getCommonSuperClass(String, String), that of this ClassWriter's runtime type by default.

Returns:

ClassLoader