Java 字节码操作示例

package com.bigdataBC.demo;

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.CtField;
import javassist.CtMethod;

/**
 * 测试使用 javassist 生成一个新的类
 * @author bboy枫亭
 * 2020年9月27日上午9:52:43
 */
public class demo01 {
	public static void main(String[] args) throws Exception {
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.makeClass("com.bigdataBC.bean.Emp");
		
		// 创建属性	（源码级操作）
		CtField f1 = CtField.make("private int empno;", cc);
		CtField f2 = CtField.make("private String ename;", cc);
		cc.addField(f2);
		cc.addField(f1);
		
		CtMethod m1 = CtMethod.make("public int getEmpno(){return empno;}", cc);
		CtMethod m2 = CtMethod.make("public void setEmpno(int empno){this.empno=empno;}", cc);
		cc.addMethod(m1);
		cc.addMethod(m2);
		
		// 添加构造器
		CtConstructor constructor = new CtConstructor(new CtClass[]{CtClass.intType,pool.get("java.lang.String")}, cc);
		constructor.setBody("{this.empno=empno;this.ename=ename;}");
		cc.addConstructor(constructor);
		
		cc.writeFile("d:/myjava");	// 将上面构造好的类写入到d:/myjava中
		System.out.println("生成类成功");
		
	}
}

package com.bigdataBC.demo;

import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.Arrays;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.CtField;
import javassist.CtMethod;
import javassist.CtNewMethod;
import javassist.NotFoundException;

/**
 * 测试javassist的API
 * @author bboy枫亭
 * 2020年9月27日上午11:13:20
 */
public class demo02 {
	// 处理类的基本用法
	public static void test01() throws Exception {
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.get("com.bigdataBC.demo.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()); // 获取父类
		System.out.println(cc.getInterfaces()); // 获得接口
	}
	// 测试产生新的方法
	public static void test02() throws Exception {
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.get("com.bigdataBC.demo.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+$2;}"); //占位符$
		
		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, 200,300);
		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(9, "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, 300);
	}

	/**
	 * 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[0]; 
		 String name = a.name();
		 int year = a.year();
		 System.out.println("name: " + name + ", year: " + year);

	}
	
	public static void main(String[] args) throws Exception {
		test02();
	}
}

package com.bigdataBC.demo;
public @interface Author { 
          String name(); 
           int year();
 }

package com.bigdataBC.demo;

public class Emp {
	private int empno;
	private String ename;
	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() {
	}
	public Emp(int empno, String ename) {
		super();
		this.empno = empno;
		this.ename = ename;
	}
	
	
}