0 Java字节码操作简介:
1 JAVAssist简介:
2 JAVAssist好处在于 在源码级别可以定义,不需要学习JVM指令
代码:
import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.CtField;
import javassist.CtMethod;
/**
* 可以使用XJad来打开class文件
* 测试使用javassist生成一个新的类 class文件
* 需要下载 javassist jar包
* @author
*
*/
public class Demo01 {
public static void main(String[] args) throws Exception {
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.makeClass("com.bean.Emp");
//创建属性
CtField f1 = CtField.make("private int empno;", cc);
CtField f2 = CtField.make("private String ename;", cc);
cc.addField(f1);
cc.addField(f2);
//创建方法
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("c:/myjava"); //将上面构造好的类写入到c:/myjava中 最后生成字节码
System.out.println("生成类,成功!");
}
}
API代码操作:
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;
import javassist.NotFoundException;
/**
* 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.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.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, 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.test.Emp");
CtMethod cm = cc.getDeclaredMethod("sayHello",new CtClass[]{CtClass.intType});
cm.insertBefore("System.out.println($1);System.out.println(\"start!!!\");"); // AOP 前置通知
cm.insertAt(9, "int b=3;System.out.println(\"b=\"+b);");
cm.insertAfter("System.out.println(\"end!!!\");"); // AOP 后置通知
//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.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.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.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 {
test06();
}
}