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

@(Java知識(shí)點(diǎn)總結(jié))[Java, 動(dòng)態(tài)字節(jié)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(zhuǎn)化為字節(jié)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(dòng)態(tài)生成一個(gè)方法，并調(diào)用
  public static void test2(ClassPool pool, CtClass cc) {
 
    try {
     /*當(dāng)CtClass對象通過writeFile()、toClass()、toBytecode()轉(zhuǎn)化為Class后，
      * Javassist凍結(jié)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(jìn)行修改，必須對其進(jìn)行解凍，通過defrost()方法進(jìn)行*/
     cc.defrost();
     // 生成一個(gè)方法
     CtMethod add = CtNewMethod.make("public int add(int a,int b){return $1+$2;}", cc);
     // 生成一個(gè)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(diào)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(zhí)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(diào)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(gòu)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(gòu)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);
  }
}