摘要：因此，若想對對象進行修改，必須對其進行解凍，通過方法進行生成一個方法生成一個方法通過反射調用生成的方法方法體前面和后面加執行語句方法體前面通過反射調用生成的方法張三生成屬性和方法獲取屬性生成構造器可以在構造器前后加代碼注解操作

@(Java知識點總結)[Java, 動態字節碼操作]

參考文章： 傳送

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;
 
/**
 * 測試 javassist 的 API
 *
 * @author Administrator
 *
 */
public class Test11 {
 
  // 通過類名加載已有的類
  public static void test1(ClassPool pool, CtClass cc) {
 
    try {
     cc = pool.get("com.gs.Emp");
 
     byte[] bytes = cc.toBytecode(); // 轉化為字節碼
     System.out.println(Arrays.toString(bytes));
 
     System.out.println(cc.getName()); // 獲取類名
     System.out.println(cc.getSimpleName());
     System.out.println(cc.getSuperclass()); // 獲取父類
     System.out.println(cc.getInterfaces()); // 獲取接口
     
    } catch (Exception e) {
     e.printStackTrace();
    }
 
  }
 
  // 動態生成一個方法，并調用
  public static void test2(ClassPool pool, CtClass cc) {
 
    try {
     /*當CtClass對象通過writeFile()、toClass()、toBytecode()轉化為Class后，
      * Javassist凍結了CtClass對象，因此，JVM不允許再次加載Class文件，所以不允許對其修改。
     因此，若想對CtClass對象進行修改，必須對其進行解凍，通過defrost()方法進行*/
     cc.defrost();
     // 生成一個方法
     CtMethod add = CtNewMethod.make("public int add(int a,int b){return $1+$2;}", cc);
     // 生成一個方法
     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);
 
     // 通過反射調用生成的方法
     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) {
    }
  }
 
  // 方法體前面和后面加執行語句
  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("方法體前面");");
     m1.insertAfter("System.out.println($1);");
 
     // 通過反射調用生成的方法
     Class clazz = cc.toClass();
     Emp emp = (Emp) clazz.newInstance();
     Method method = clazz.getDeclaredMethod("sayHello", new Class[] { String.class });
     method.invoke(emp, "張三");
 
    } catch (Exception e) {
     e.printStackTrace();
    }
 
  }
 
  // 生成屬性和方法
  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"); //獲取屬性
 
     CtMethod method = CtNewMethod.getter("getSalary", field);
     cc.addMethod(method);
 
     CtMethod method2 = CtNewMethod.getter("setSalary", field);
     cc.addMethod(method2);
    } catch (Exception e) {
     e.printStackTrace();
    }
 
  }
 
  // 生成構造器
  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("可以在構造器前后加代碼");");
     }
    } 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);
  }
}