使用ASM动态生成一个接口的实现类，接口如下：
public interface ISayHello { public void MethodA(); public void MethodB(); public void Abs(); }
具体实现如下：
public class InterfaceHandler extends ClassLoader implements Opcodes { public static Object MakeClass(Class<?> clazz) throws Exception { String name = clazz.getSimpleName(); String className = name + "$imp"; String Iter = clazz.getName().replaceAll("\\.", "/"); ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS); cw.visit(V1_5, ACC_PUBLIC + ACC_SUPER, className, null, "java/lang/Object", new String[] { Iter }); // 空构造 MethodVisitor mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null); mv.visitVarInsn(ALOAD, 0); mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V"); mv.visitInsn(RETURN); mv.visitMaxs(1, 1); mv.visitEnd(); // 实现接口中所有方法 Method[] methods = clazz.getMethods(); for (Method method : methods) { MakeMethod(cw, method.getName(), className); } cw.visitEnd(); //写入文件 byte[] code = cw.toByteArray(); FileOutputStream fos = new FileOutputStream("d:/com/" + className + ".class"); fos.write(code); fos.close(); //从文件加载类 InterfaceHandler loader = new InterfaceHandler(); Class<?> exampleClass = loader.defineClass(className, code, 0, code.length); Object obj = exampleClass.getConstructor().newInstance(); return obj; } private static void MakeMethod(ClassWriter cw, String MethodName, String className) { MethodVisitor mv = cw.visitMethod(ACC_PUBLIC, MethodName, "()V", null, null); //mv.visitCode(); //Label l0 = new Label(); //mv.visitLabel(l0); //mv.visitLineNumber(8, l0); mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;"); mv.visitLdcInsn("调用方法 [" + MethodName + "]"); mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V"); //Label l1 = new Label(); //mv.visitLabel(l1); //mv.visitLineNumber(9, l1); mv.visitInsn(RETURN); //Label l2 = new Label(); //mv.visitLabel(l2); //mv.visitLocalVariable("this", "L" + className + ";", null, l0, l2, 0); mv.visitMaxs(2, 1); mv.visitEnd(); } public static void main(final String args[]) throws Exception { ISayHello iSayHello = (ISayHello) MakeClass(ISayHello.class); iSayHello.MethodA(); iSayHello.MethodB(); iSayHello.Abs(); } }
注意，使用ASM访问属性和方法的时候，会返回一个Visitor对象，如属性为FieldVisitor，方法为MethodVisitor。
使用反编译工具查看生成的字节码文件内容如下：
public class ISayHello$imp implements ISayHello { public void MethodA() { System.out.println("调用方法 [MethodA]"); } public void MethodB() { System.out.println("调用方法 [MethodB]"); } public void Abs() { System.out.println("调用方法 [Abs]"); } }
使用ASM生成接口实现类
在java的许多框架里面，都能找到ASM的身影，比如AOP编程就可以利visitMethod对指定方法就行拦截，做前置后置增强，还有比如常用的插件Lombok就是利用ASM添加的setter getter方法。MyBatis的Mapper接口实现是通过动态代理实现，现在可以使用ASM动态创建实现了字节码来实现。
1、定义接口
定义StudentMapper接口
package org.example.cn.mapper; import java.util.List; public interface StudentMapper { List<String> findStudentList(); }
2、生成实现类
使用ASM生成实现类StudentMapperImpl
ClassWriter classWriter = new ClassWriter(0); /// 参数列表第3个参数表示继承的父类，第4个参数表示实现的接口 classWriter.visit(61, ACC_PUBLIC | ACC_SUPER, "org/example/cn/mapper/StudentMapperImpl", null, "java/lang/Object", new String[]{"org/example/cn/mapper/StudentMapper"}); classWriter.visitSource("StudentMapperImpl.java", null); /// 定义构造器 MethodVisitor methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null); methodVisitor.visitCode(); Label initLabel0 = new Label(); methodVisitor.visitLabel(initLabel0); methodVisitor.visitVarInsn(ALOAD, 0); methodVisitor.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V"); methodVisitor.visitInsn(RETURN); Label initLabel1 = new Label(); methodVisitor.visitLabel(initLabel1); /// 局部变量表，槽位的0处放的是this变量 methodVisitor.visitLocalVariable("this", "Lorg/example/cn/mapper/StudentMapperImpl;", null, initLabel0, initLabel1, 0); methodVisitor.visitMaxs(1, 1); methodVisitor.visitEnd(); /// 重写findStudentList方法 methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "findStudentList", "()Ljava/util/List;", "()Ljava/util/List<Ljava/lang/String;>;", null); methodVisitor.visitCode(); Label label0 = new Label(); methodVisitor.visitLabel(label0); /// new ArrayList() methodVisitor.visitTypeInsn(NEW, "java/util/ArrayList"); methodVisitor.visitInsn(DUP); methodVisitor.visitMethodInsn(INVOKESPECIAL, "java/util/ArrayList", "<init>", "()V"); methodVisitor.visitVarInsn(ASTORE, 1); Label label1 = new Label(); methodVisitor.visitLabel(label1); /// 压栈 methodVisitor.visitVarInsn(ALOAD, 1); /// 赋值 methodVisitor.visitLdcInsn("\u9648\u7476"); ///调用add方法 methodVisitor.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z"); /// 出栈 methodVisitor.visitInsn(POP); Label label2 = new Label(); methodVisitor.visitLabel(label2); methodVisitor.visitVarInsn(ALOAD, 1); methodVisitor.visitLdcInsn("\u674e\u73b0"); methodVisitor.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z"); methodVisitor.visitInsn(POP); Label label3 = new Label(); methodVisitor.visitLabel(label3); methodVisitor.visitLineNumber(11, label3); methodVisitor.visitVarInsn(ALOAD, 1); methodVisitor.visitLdcInsn("\u91d1\u6668"); methodVisitor.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z"); methodVisitor.visitInsn(POP); Label label4 = new Label(); methodVisitor.visitLabel(label4); methodVisitor.visitLineNumber(12, label4); methodVisitor.visitVarInsn(ALOAD, 1); methodVisitor.visitInsn(ARETURN); Label label5 = new Label(); methodVisitor.visitLabel(label5); methodVisitor.visitLocalVariable("this", "Lorg/example/cn/mapper/StudentMapperImpl;", null, label0, label5, 0); methodVisitor.visitLocalVariable("list", "Ljava/util/List;", "Ljava/util/List<Ljava/lang/String;>;", label1, label5, 1); methodVisitor.visitMaxs(2, 2); methodVisitor.visitEnd(); classWriter.visitEnd(); byte[] bytes = classWriter.toByteArray();
字节码java源码
public class StudentMapperImpl implements StudentMapper { public List<String> findStudentList() { List<String> list = new ArrayList(); list.add("陈瑶"); list.add("李现"); list.add("金晨"); return list; } }
3、加载实现类
自定义类加载器加载StudentMapperImpl字节码
public class MyClassLoader extends ClassLoader{ /// 类名全路径 private final String className; /// 字节码 private final byte[] bytes; public MyClassLoader( String className,byte[] bytes){ this.className = className; this.bytes = bytes; } @Override protected Class<?> findClass(String name) throws ClassNotFoundException { return defineClass(className,bytes,0,bytes.length); } }
加载类
String className = "org.example.cn.mapper.StudentMapperImpl"; MyClassLoader myClassLoader = new MyClassLoader(className, bytes); Class<?> clz = myClassLoader.loadClass(className);
4、调用实现类
◆ 方法1
利用构造器直接反射创建实例
/// Class<?> aClass = Class.forName("org.example.cn.mapper.StudentMapperImpl"); StudentMapper studentMapper = (StudentMapper) clz.getConstructor().newInstance(); List<String> studentList = studentMapper.findStudentList(); System.out.println("studentList---"+studentList);
方法2
动态代理实例化接口
StudentMapper studentMapper = (StudentMapper) Proxy.newProxyInstance(myClassLoader, new Class[]{StudentMapper.class}, (proxy, method, args1) -> method.invoke(clz.getConstructor().newInstance(), args1)); List<String> studentList = studentMapper.findStudentList(); System.out.println("studentList---"+studentList);
运行结果
