常见的Java字节码增强框架有两种:ASM和Javassist。
ASM框架使用攻略
1. 引入ASM库
在Maven项目中,在pom.xml
文件中添加如下依赖即可:
<dependency>
<groupId>org.ow2.asm</groupId>
<artifactId>asm</artifactId>
<version>7.2</version>
</dependency>
2. 编写ClassVisitor
ClassVisitor是ASM框架中的主要接口之一,用于对Class文件进行访问和修改。下面的示例将对Class文件中的每一个方法进行访问和修改:
public class MyClassVisitor extends ClassVisitor {
public MyClassVisitor(int api, ClassVisitor cv) {
super(api, cv);
}
@Override
public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
return new MyMethodVisitor(api, mv);
}
}
3. 编写MethodVisitor
MethodVisitor是ASM框架中的另一个主要接口,用于对Method进行访问和修改。下面的示例将替换方法中的指令:
public class MyMethodVisitor extends MethodVisitor {
public MyMethodVisitor(int api, MethodVisitor mv) {
super(api, mv);
}
@Override
public void visitInsn(int opcode) {
if (opcode == Opcodes.RETURN) {
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitLdcInsn("Hello World!");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
}
}
}
4. 使用ASM增强Java类
下面的示例展示了如何使用ASM框架增强Java类:
public class MyClassTransformer implements ClassFileTransformer {
@Override
public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) {
ClassReader cr = new ClassReader(classfileBuffer);
ClassWriter cw = new ClassWriter(cr, ClassWriter.COMPUTE_MAXS | ClassWriter.COMPUTE_FRAMES);
MyClassVisitor cv = new MyClassVisitor(Opcodes.ASM7, cw);
cr.accept(cv, ClassReader.EXPAND_FRAMES);
return cw.toByteArray();
}
}
Javassist框架使用攻略
1. 引入Javassist库
在Maven项目中,在pom.xml
文件中添加如下依赖即可:
<dependency>
<groupId>org.javassist</groupId>
<artifactId>javassist</artifactId>
<version>3.27.0-GA</version>
</dependency>
2. 使用Javassist增强Java类
下面的示例展示了如何使用Javassist框架增强Java类:
public class MyClassTransformer implements ClassFileTransformer {
@Override
public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) {
ClassPool cp = ClassPool.getDefault();
CtClass cc = cp.makeClass(new ByteArrayInputStream(classfileBuffer));
try {
CtMethod m = cc.getDeclaredMethod("myMethod");
m.insertBefore("System.out.println(\"Before\");");
m.insertAfter("System.out.println(\"After\");");
} catch (NotFoundException | CannotCompileException e) {
e.printStackTrace();
}
return cc.toBytecode();
}
}
3. 使用Javassist动态创建Java类
下面的示例展示了如何使用Javassist框架动态创建Java类:
public class MyClassCreator {
public static void main(String[] args) throws Exception {
ClassPool cp = ClassPool.getDefault();
CtClass cc = cp.makeClass("MyClass");
CtField f = CtField.make("private int x = 0;", cc);
cc.addField(f);
CtMethod m = CtNewMethod.make("public void add(int num) { x += num; }", cc);
cc.addMethod(m);
Class<?> c = cc.toClass();
Object o = c.newInstance();
Method addMethod = c.getDeclaredMethod("add", int.class);
addMethod.invoke(o, 1);
Field xField = c.getDeclaredField("x");
xField.setAccessible(true);
System.out.println(xField.get(o));
}
}
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