Java知识点总结（动态字节码操作-Javassist的API使用）

摘要：因此，若想对对象进行修改，必须对其进行解冻，通过方法进行生成一个方法生成一个方法通过反射调用生成的方法方法体前面和后面加执行语句方法体前面通过反射调用生成的方法张三生成属性和方法获取属性生成构造器可以在构造器前后加代码注解操作

@(Java知识点总结)[Java, 动态字节码操作]

参考文章： 传送

操作示例：

public @interface Author {
  String name();
  int year();
}

@Author(name = "gs",year=2015)
public class Emp {
  private int num;
  private String name;
  
  public Emp() {
  }
  
  public Emp(int num, String name) {
   this();
   this.num = num;
   this.name = name;
  }
  
  public void sayHello(String name){
   System.out.print("你好,");
  }
  public int getNum() {
   return num;
  }
  public void setNum(int num) {
   this.num = num;
  }
  public String getName() {
   return name;
  }
  public void setName(String name) {
   this.name = name;
  }
  
}

 
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 Administrator
 *
 */
public class Test11 {
 
  // 通过类名加载已有的类
  public static void test1(ClassPool pool, CtClass cc) {
 
    try {
     cc = pool.get("com.gs.Emp");
 
     byte[] bytes = cc.toBytecode(); // 转化为字节码
     System.out.println(Arrays.toString(bytes));
 
     System.out.println(cc.getName()); // 获取类名
     System.out.println(cc.getSimpleName());
     System.out.println(cc.getSuperclass()); // 获取父类
     System.out.println(cc.getInterfaces()); // 获取接口
     
    } catch (Exception e) {
     e.printStackTrace();
    }
 
  }
 
  // 动态生成一个方法，并调用
  public static void test2(ClassPool pool, CtClass cc) {
 
    try {
     /*当CtClass对象通过writeFile()、toClass()、toBytecode()转化为Class后，
      * Javassist冻结了CtClass对象，因此，JVM不允许再次加载Class文件，所以不允许对其修改。
     因此，若想对CtClass对象进行修改，必须对其进行解冻，通过defrost()方法进行*/
     cc.defrost();
     // 生成一个方法
     CtMethod add = CtNewMethod.make("public int add(int a,int b){return $1+$2;}", cc);
     // 生成一个方法
     CtMethod subtraction = new CtMethod(CtClass.intType, "subtraction",
         new CtClass[] { CtClass.intType, CtClass.intType }, cc);
     subtraction.setModifiers(Modifier.PUBLIC);
     subtraction.setBody("return $1-$2;");
 
     cc.addMethod(add);
     cc.addMethod(subtraction);
 
     // 通过反射调用生成的方法
     Class clazz = cc.toClass();
     Emp emp = (Emp) clazz.newInstance();
     // Method method = clazz.getDeclaredMethod("add", new
     // Class[]{int.class,int.class});
     Method method = clazz.getDeclaredMethod("subtraction", new Class[] { int.class, int.class });
     Object result = method.invoke(emp, 200, 300);
     System.out.println(result);
    } catch (Exception e) {
    }
  }
 
  // 方法体前面和后面加执行语句
  public static void test3(ClassPool pool, CtClass cc) {
 
    try {
     cc.defrost();
     CtMethod m1 = cc.getDeclaredMethod("sayHello", new CtClass[] { pool.get("java.lang.String") });
     m1.insertBefore("System.out.println("方法体前面");");
     m1.insertAfter("System.out.println($1);");
 
     // 通过反射调用生成的方法
     Class clazz = cc.toClass();
     Emp emp = (Emp) clazz.newInstance();
     Method method = clazz.getDeclaredMethod("sayHello", new Class[] { String.class });
     method.invoke(emp, "张三");
 
    } catch (Exception e) {
     e.printStackTrace();
    }
 
  }
 
  // 生成属性和方法
  public static void test4(ClassPool pool, CtClass cc) {
    try {
     // CtField.make("private double salary;", cc);
     CtField field = new CtField(CtClass.doubleType, "salary", cc);
     field.setModifiers(Modifier.PRIVATE);
     cc.addField(field);
 
     // cc.getDeclaredField("salary"); //获取属性
 
     CtMethod method = CtNewMethod.getter("getSalary", field);
     cc.addMethod(method);
 
     CtMethod method2 = CtNewMethod.getter("setSalary", field);
     cc.addMethod(method2);
    } catch (Exception e) {
     e.printStackTrace();
    }
 
  }
 
  // 生成构造器
  public static void test5(ClassPool pool, CtClass cc) {
 
    try {
     cc.defrost();
     CtConstructor[] cs = cc.getConstructors();
     for (CtConstructor c : cs) {
       System.out.println(c.getLongName());
       c.insertAfter("System.out.println("可以在构造器前后加代码");");
     }
    } catch (Exception e) {
     e.printStackTrace();
    }
 
  }
 
  // 注解操作
  public static void test6(ClassPool pool, CtClass cc) {
    try {
     cc.defrost();
     Object[] annotations = cc.getAnnotations();
     Author author = (Author) annotations[0];
     System.out.println("name:" + author.name() + ",year:" + author.year());
 
    } catch (Exception e) {
     e.printStackTrace();
    }
  }
 
  public static void main(String[] args) {
    ClassPool pool = ClassPool.getDefault();
    CtClass cc = null;
    try {
     cc = pool.get("com.gs.Emp");
    } catch (NotFoundException e) {
     e.printStackTrace();
    }
 
    test1(pool, cc);
    //test2(pool, cc);
    test3(pool, cc);
    test5(pool, cc);
    test6(pool, cc);
  }
}