public class

ClassWriter

extends Object
implements ClassVisitor
java.lang.Object
   ↳ org.sonatype.guice.bean.scanners.asm.ClassWriter

Class Overview

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.

Summary

Constants
int COMPUTE_FRAMES Flag to automatically compute the stack map frames of methods from scratch.
int COMPUTE_MAXS Flag to automatically compute the maximum stack size and the maximum number of local variables of methods.
Public Constructors
ClassWriter(int flags)
Constructs a new ClassWriter object.
ClassWriter(ClassReader classReader, int flags)
Constructs a new ClassWriter object and enables optimizations for "mostly add" bytecode transformations.
Public Methods
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.
Protected Methods
String getCommonSuperClass(String type1, String type2)
Returns the common super type of the two given types.
[Expand]
Inherited Methods
From class java.lang.Object
From interface org.sonatype.guice.bean.scanners.asm.ClassVisitor

Constants

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 visitFrame(int, int, Object[], int, 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
Constant Value: 2 (0x00000002)

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
Constant Value: 1 (0x00000001)

Public Constructors

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.

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.

Public Methods

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.

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.

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.

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.

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.

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.

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.

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

Visits the header of the class.

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.

public AnnotationVisitor visitAnnotation (String desc, boolean visible)

Visits an annotation of the class.

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.

public void visitAttribute (Attribute attr)

Visits a non standard attribute of the class.

Parameters
attr an attribute.

public void visitEnd ()

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.

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

Visits a field of the class.

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.

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

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

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.

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

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.

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.

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

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

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.

public void visitSource (String file, String debug)

Visits the source of the class.

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.

Protected Methods

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.