学习笔记——JAVA字节码操作 Javassist的用法

功能 动态生成新的类 动态改变某个类的结构（增删改查 新的属性和方法）

优势 比反射开销小 javaAasist性能高于反射低于ASM

常见的类库 BCEL ASM CGLIB Javassist//下载地址http://jboss-javassist.github.io/javassist/

javassist主要有CtClass,CtMethod,CtFile。用于执行和jdk反射API中的java.lang.Class,java.lang.reflect.Method,java.lang.reflect.Method.Field相同的操作

创建一个类

package study; import java.io.IOException; import javassist.CannotCompileException; import javassist.ClassPool; import javassist.CtClass; import javassist.CtConstructor; import javassist.CtField; import javassist.CtMethod; import javassist.NotFoundException; /** * 测试javassist生成一个新的类 * * @author http://blog.csdn.net/thewaiting * */ public class Text { public static void main(String[] args) throws CannotCompileException, NotFoundException, IOException { ClassPool pool = ClassPool.getDefault(); CtClass cc = pool.makeClass("study2.Person"); // 创建属性 CtField f = CtField.make("private int id;", cc); CtField f2 = CtField.make("private String name;", cc); cc.addField(f); cc.addField(f2); // 创建方法 CtMethod m = CtMethod.make("public int getId(){return this.id;}", cc); CtMethod m2 = CtMethod.make("public void setId(int id){this.id = id;}", cc); cc.addMethod(m); cc.addMethod(m2); // 添加构造器 CtConstructor c = new CtConstructor(new CtClass[] { CtClass.intType, pool.get("java.lang.String") }, cc); // 构造方法体 c.setBody("{this.id = id;this.name = name;}"); cc.addConstructor(c); // 放入工作空间 cc.writeFile("D:/text"); System.out.println("生成的类用反编译工具查看"); } }

获取一个类

package study; public class Person { private int id; private String name; public Person() { } public Person(int id, String name) { super(); this.id = id; this.name = name; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } }

javassist的用法

package study; import java.io.IOException; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.security.AllPermission; import java.util.Arrays; import javassist.CannotCompileException; import javassist.ClassPool; import javassist.CtBehavior; import javassist.CtClass; import javassist.CtConstructor; import javassist.CtField; import javassist.CtMember; import javassist.CtMethod; import javassist.CtNewMethod; import javassist.Modifier; import javassist.NotFoundException; /** * * 测试javassist的api * * @author http://blog.csdn.net/thewaiting * */ public class Text { /** * 处理类的基本用法 * * @throws NotFoundException * @throws CannotCompileException * @throws IOException */ public static void text1() throws NotFoundException, IOException, CannotCompileException { // 获得池对象 ClassPool pool = ClassPool.getDefault(); // 获得指定某个类 CtClass cc = pool.get("study.Person"); byte[] bytes = cc.toBytecode(); System.out.println(Arrays.toString(bytes)); System.out.println(cc.getName());// 获得类名 System.out.println(cc.getSimpleName()); System.out.println(cc.getSuperclass());// 获得父类 } /** * 测试产生新的方法 * * @throws NotFoundException * @throws CannotCompileException * @throws IllegalAccessException * @throws InstantiationException * @throws SecurityException * @throws NoSuchMethodException * @throws InvocationTargetException * @throws IllegalArgumentException */ public static void text2() throws NotFoundException, CannotCompileException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException { // 获得池对象 ClassPool pool = ClassPool.getDefault(); // 获得指定某个类 CtClass cc = pool.get("study.Person"); // CtMember m = CtNewMethod.make("public int add(int a,int b){return // a+b}", cc); // 参数 指定返回类型 方法的名称 方法的参数 CtMember m = new CtMethod(CtClass.intType, "add", new CtClass[] { CtClass.intType, CtClass.intType }, cc); // 是什么方法 m.setModifiers(Modifier.PUBLIC); ((CtBehavior) m).setBody("{return $1+$2;}");// $0 为this $后为形参 cc.addMethod((CtMethod) m); // 通过反射调用新生成的方法 Class clazz = cc.toClass(); Object obj = clazz.newInstance();// 通过调用person无参构造，创建新的person对象 Method method = clazz.getDeclaredMethod("add", int.class, int.class); System.out.println(method.invoke(obj, 200, 300)); } /** * 修改方法已经有的信息 * * @throws NotFoundException * @throws CannotCompileException * @throws NoSuchMethodException * @throws SecurityException * @throws InstantiationException * @throws IllegalAccessException * @throws IllegalArgumentException * @throws InvocationTargetException */ public static void text3() throws NotFoundException, CannotCompileException, NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException { // 获得池对象 ClassPool pool = ClassPool.getDefault(); // 获得指定某个类 CtClass cc = pool.get("study.Person"); CtMethod cm = cc.getDeclaredMethod("say", new CtClass[] { CtClass.intType });// 第二个参数为数组 // 修改方法 在方法前加 在方法的第几行加 在方法后面加 cm.insertBefore("System.out.println($1+\" start\");"); cm.insertAt(8, "int b = 3;System.out.println(b);"); cm.insertAfter("System.out.println($1+\" end\");"); // 通过反射调用新生成的方法 Class clazz = cc.toClass(); Object obj = clazz.newInstance(); Method m = clazz.getDeclaredMethod("say", int.class); System.out.println(m.invoke(obj, 12)); } /** * 属性的操作 * * @throws NotFoundException * @throws CannotCompileException */ public static void text4() throws NotFoundException, CannotCompileException { // 获得池对象 ClassPool pool = ClassPool.getDefault(); // 获得指定某个类 CtClass cc = pool.get("study.Person"); // CtField field = CtField.make("private int id;", cc); CtField f1 = new CtField(CtClass.intType, "age", cc); f1.setModifiers(Modifier.PRIVATE); // 加上值 cc.addField(f1, "10"); // 获取值 // cc.getDeclaredField("name"); // 增加get set 方法 cc.addMethod(CtNewMethod.getter("getAge", f1)); cc.addMethod(CtNewMethod.getter("setAge", f1)); } /** * 构造方法的操作 * * @throws NotFoundException */ public static void text5() throws NotFoundException { // 获得池对象 ClassPool pool = ClassPool.getDefault(); // 获得指定某个类 CtClass cc = pool.get("study.Person"); CtConstructor[] cs = cc.getConstructors(); for (CtConstructor c : cs) { System.out.println(c.getLongName()); //c.insertBefore("");添加方法 } } public static void text6() throws NotFoundException, ClassNotFoundException { CtClass cc = ClassPool.getDefault().get("study.Person"); Object[] all = cc.getAnnotations(); Author a = (Author) all[0]; String name = a.name(); int year = a.year(); System.out.println("name "+name + " year "+ year); //能进一步操作 } public static void main(String[] args) throws NotFoundException, IOException, CannotCompileException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException, ClassNotFoundException { // text01(); // text2(); // text3(); // text4(); // text5(); text6(); } }