字节码操作-Javaassist

下面就是一个具体的demo来介绍利用Javaassist库来创建类，不过要先在工程里面导入Javaassist的架包，

package JavaAasist;
 
import java.lang.reflect.Method;
import java.util.Arrays;
 
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.CtField;
import javassist.CtMethod;
import javassist.CtNewMethod;
import javassist.Modifier;
 
/**
* 测试javassist的API
* @author 尚学堂高淇 www.sxt.cn
*
*/
public class DEmo02 {
/**
* 处理类的基本用法
* @throws Exception
*/
public static void test01() throws Exception{
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("com.bjsxt.test.Emp");
 
byte[] bytes = cc.toBytecode();
System.out.println(Arrays.toString(bytes));
 
System.out.println(cc.getName()); //获取类名
System.out.println(cc.getSimpleName()); //获取简要类名
System.out.println(cc.getSuperclass()); //获得父类
System.out.println(cc.getInterfaces()); //获得接口
 
}
 
/**
* 测试产生新的方法
* @throws Exception
*/
public static void test02() throws Exception{
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("com.bjsxt.test.Emp");
 
//    CtMethod m = CtNewMethod.make("public int add(int a,int b){return a+b;}", cc);
 
CtMethod m = new CtMethod(CtClass.intType,"add",
new CtClass[]{CtClass.intType,CtClass.intType},cc);
m.setModifiers(Modifier.PUBLIC);
m.setBody("{System.out.println(\"www.sxt.cn\");return $1+$2;}");
 
cc.addMethod(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Emp无参构造器，创建新的Emp对象
Method method = clazz.getDeclaredMethod("add", int.class,int.class);
Object result = method.invoke(obj, ,);
System.out.println(result);
}
 
/**
* 修改已有的方法的信息，修改方法体的内容
* @throws Exception
*/
public static void test03() throws Exception{
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("com.bjsxt.test.Emp");
 
CtMethod cm = cc.getDeclaredMethod("sayHello",new CtClass[]{CtClass.intType});
cm.insertBefore("System.out.println($1);System.out.println(\"start!!!\");");
cm.insertAt(, "int b=3;System.out.println(\"b=\"+b);");
cm.insertAfter("System.out.println(\"end!!!\");");
 
//通过反射调用新生成的方法
Class clazz = cc.toClass();
Object obj = clazz.newInstance(); //通过调用Emp无参构造器，创建新的Emp对象
Method method = clazz.getDeclaredMethod("sayHello", int.class);
method.invoke(obj, );
}
 
/**
* 属性的操作
* @throws Exception
*/
public static void test04() throws Exception{
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("com.bjsxt.test.Emp");
 
//    CtField f1 = CtField.make("private int empno;", cc);
CtField f1 = new CtField(CtClass.intType,"salary",cc);
f1.setModifiers(Modifier.PRIVATE);
cc.addField(f1);
 
//    cc.getDeclaredField("ename"); //获取指定的属性
 
//增加相应的set和get方法
cc.addMethod(CtNewMethod.getter("getSalary", f1));;
cc.addMethod(CtNewMethod.getter("setSalary", f1));;
 
}
 
/**
* 构造方法的操作
* @throws Exception
*/
public static void test05() throws Exception {
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("com.bjsxt.test.Emp");
 
CtConstructor[] cs = cc.getConstructors();
for (CtConstructor c : cs) {
System.out.println(c.getLongName());
}
}
 
public static void test06() throws Exception{
CtClass cc = ClassPool.getDefault().get("com.bjsxt.test.Emp");
Object[] all = cc.getAnnotations();
Author a = (Author)all[];
String name = a.name();
int year = a.year();
System.out.println("name: " + name + ", year: " + year);
 
}
 
public static void main(String[] args) throws Exception {
test06();
}
}

运行改代码则在对应路径生成一个java文件，不过该文件需要反编译才可以查看，具体的工具是xjad

下面是JavaAssist处理类的demo

package JavaAssist;
 
import java.lang.reflect.Method;
import java.util.Arrays;
 
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.CtField;
import javassist.CtMethod;
import javassist.CtNewMethod;
import javassist.Modifier;
 
/**
* 测试javassist的API
* @author 19579
*
*/
public class DEmo02 {
/**
* 处理类的基本用法
* @throws Exception
*/
public static void test01() throws Exception{
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("JavaAssist.Emp");
 
byte[] bytes = cc.toBytecode();
System.out.println(Arrays.toString(bytes));
 
System.out.println(cc.getName()); //获取类名
System.out.println(cc.getSimpleName()); //获取简要类名
System.out.println(cc.getSuperclass()); //获得父类
System.out.println(cc.getInterfaces()); //获得接口
 
}
 
/**
* 测试产生新的方法
* @throws Exception
*/
public static void test02() throws Exception{
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("JavaAssist.Emp");
 
//    CtMethod m = CtNewMethod.make("public int add(int a,int b){return a+b;}", cc);
 
CtMethod m = new CtMethod(CtClass.intType,"add",
new CtClass[]{CtClass.intType,CtClass.intType},cc);
m.setModifiers(Modifier.PUBLIC);
m.setBody("{System.out.println(\"www.sxt.cn\");return $1+$2;}");//$1和$2分别指两个形参
 
cc.addMethod(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Emp无参构造器，创建新的Emp对象
Method method = clazz.getDeclaredMethod("add", int.class,int.class);
Object result = method.invoke(obj, ,);
System.out.println(result);
}
 
/**
* 修改已有的方法的信息，修改方法体的内容
* @throws Exception
*/
public static void test03() throws Exception{
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("JavaAssist.Emp");
 
CtMethod cm = cc.getDeclaredMethod("sayHello",new CtClass[]{CtClass.intType});
cm.insertBefore("System.out.println($1);System.out.println(\"start!!!\");");//在方法体前加入
cm.insertAt(, "int b=3;System.out.println(\"b=\"+b);");//具体的第9行加入
cm.insertAfter("System.out.println(\"end!!!\");");//在方法后面加入
 
//通过反射调用新生成的方法
Class clazz = cc.toClass();
Object obj = clazz.newInstance(); //通过调用Emp无参构造器，创建新的Emp对象
Method method = clazz.getDeclaredMethod("sayHello", int.class);
method.invoke(obj, );
}
 
/**
* 属性的操作
* @throws Exception
*/
public static void test04() throws Exception{
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("JavaAssist.Emp");
 
//    CtField f1 = CtField.make("private int empno;", cc);
CtField f1 = new CtField(CtClass.intType,"salary",cc);
f1.setModifiers(Modifier.PRIVATE);
cc.addField(f1);
 
//    cc.getDeclaredField("ename"); //获取指定的属性
 
//增加相应的set和get方法
cc.addMethod(CtNewMethod.getter("getSalary", f1));;
cc.addMethod(CtNewMethod.getter("setSalary", f1));;
 
}
 
/**
* 构造方法的操作
* @throws Exception
*/
public static void test05() throws Exception {
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("JavaAssist.Emp");
 
CtConstructor[] cs = cc.getConstructors();
for (CtConstructor c : cs) {
System.out.println(c.getLongName());
}
}
 
public static void test06() throws Exception{
CtClass cc = ClassPool.getDefault().get("JavaAssist.Emp");
Object[] all = cc.getAnnotations();
Author a = (Author)all[];
String name = a.name();
int year = a.year();
System.out.println("name: " + name + ", year: " + year);
 
}
 
public static void main(String[] args) throws Exception {
test06();
}
}

对应的emp类如下

package com.bjsxt.test;
 
@Author(name="gaoqi", year=)
public class Emp {
 
private int empno;
private String ename;
 
public void sayHello(int a){
System.out.println("sayHello,"+a);
}
 
public int getEmpno() {
return empno;
}
public void setEmpno(int empno) {
this.empno = empno;
}
public String getEname() {
return ename;
}
public void setEname(String ename) {
this.ename = ename;
}
 
public Emp(int empno, String ename) {
super();
this.empno = empno;
this.ename = ename;
}
 
public Emp() {
}
}
 
Author类如下
 
package com.bjsxt.test;
public @interface Author {
String name();
int year();
}

当然javaAssist也有自己的局限性