Java反射那点事（一）

简介

JAVA反射机制是在运行状态中，对于任意一个类，都能够知道这个类的所有属性和方法；对于任意一个对象，都能够调用它的任意一个方法和属性；这种动态获取的信息以及动态调用对象的方法的功能称为java语言的反射机制。
Java反射机制主要提供了以下功能： 在运行时判断任意一个对象所属的类；在运行时构造任意一个类的对象；在运行时判断任意一个类所具有的成员变量和方法；在运行时调用任意一个对象的方法；生成动态代理。

最近在使用JSONArray转化JSON格式的时候，总觉得它使用了Java的反射机制。在Spring中也有一些用到反射的地方，因此，先简单总结一下Java反射基础的一些东西。

代码举例：

说明：新建了一个Person接口，Student类作为测试对象。

1.Person.java

/*
 * $filename: person.java,v $
 * $Date: 2013-11-10  $
 * Copyright (C) ZhengHaibo, Inc. All rights reserved.
 * This software is Made by Zhenghaibo.
 */
package edu.njupt.zhb.reflection;
/*
 *@author: ZhengHaibo  

 *2013-11-10  Nanjing,njupt,China
 */
public interface Person {
  public void say();
}

2.Student.java

/*
 * $filename: Student.java,v $
 * $Date: 2013-11-10  $
 * Copyright (C) ZhengHaibo, Inc. All rights reserved.
 * This software is Made by Zhenghaibo.
 */
package edu.njupt.zhb.reflection;
/*
 *@author: ZhengHaibo  

 *2013-11-10  Nanjing,njupt,China
 */
@Deprecated
public class Student implements Person{
  public boolean isGood;
  private String id;
  private String name;
  public Student(String id, String name) {
    this.id = id;
    this.name = name;
  }
  public Student() {

  }
  public String getId() {
    return id;
  }
  public void setId(String id) {
    this.id = id;
  }
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  private void privateMethod(){
    System.out.println("this is a private Method!");
  }
  @Override
  public void say() {
    // TODO Auto-generated method stub
    System.out.println("I am a student,My name is "+name+",My id is "+id);
  }
}

3.测试类

/*
 * $filename: JavaReflectionTest.java,v $
 * $Date: 2013-11-10  $
 * Copyright (C) ZhengHaibo, Inc. All rights reserved.
 * This software is Made by Zhenghaibo.
 * Reference: http://tutorials.jenkov.com/java-reflection/index.html
 */
package edu.njupt.zhb.reflection;

import java.lang.annotation.Annotation;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;

/*
 *@author: ZhengHaibo  

 *2013-11-10  Nanjing,njupt,China
 */
public class JavaReflectionTest {

  /**
   * @param args
   */
  public static void main(String[] args) {
    // TODO Auto-generated method stub
    JavaReflectionTest jrt = new JavaReflectionTest();
    System.out.println("--基本信息的输出----------------------------------------------------------------");
    jrt.getBaseInfoByClass();
    System.out.println("--测试构造方法的输出----------------------------------------------------------------");
    jrt.testConstructor();// 输出：I am a student,My name is 郑海波,My id is 1012010638
    System.out.println("--测试invoke方法的输出----------------------------------------------------------------");
    jrt.testMethod();
    System.out.println("--测试获取get和set方法的输出----------------------------------------------------------------");
    jrt.printGettersSetters(Student.class);
    System.out.println("--测试访问私有成员变量和私有函数的输出--------------------------------------------------------");
    jrt.testPrivateFieldAndMethod();
  }

  /**
   * 基本信息的输出
   */
  public void getBaseInfoByClass() {
    Class<Student> studentClass = Student.class;// 获得Class对象
    Class<Student> studentClass2 = null;
    try {
      studentClass2 = (Class<Student>)Class.forName("edu.njupt.zhb.reflection.Student");
    } catch (ClassNotFoundException e2) {
      // TODO Auto-generated catch block
      e2.printStackTrace();
    }//通过forName获得Class对象
    String className = studentClass.getName();// 获得类名
    System.out.println("className=" + className);
    Field[] fields = studentClass.getFields();// 获得public成员变量
    for (Field field : fields) {
      System.out.println("field=" + field);
    }
    Method[] methods = studentClass.getMethods();// 获得成员函数
    for (Method method : methods) {
      System.out.println("method=" + method);
    }
    /**
     * Modifier.isAbstract(int modifiers) Modifier.isFinal(int modifiers)
     * Modifier.isInterface(int modifiers) Modifier.isNative(int modifiers)
     * Modifier.isPrivate(int modifiers) Modifier.isProtected(int modifiers)
     * Modifier.isPublic(int modifiers) Modifier.isStatic(int modifiers)
     * Modifier.isStrict(int modifiers) Modifier.isSynchronized(int
     * modifiers) Modifier.isTransient(int modifiers)
     * Modifier.isVolatile(int modifiers)
     */
    int modifier = studentClass.getModifiers();// 获得类的属性
    System.out.println(Modifier.isInterface(modifier));
    Package packagePath = studentClass.getPackage();// 获得类所在包
    System.out.println("packagePath=" + packagePath.getName());
    Class<Object> superclass = (Class<Object>) studentClass.getSuperclass();// 获取类的父类
    System.out.println("superclassName=" + superclass.getName());
    Class[] interfaces = studentClass.getInterfaces();// 获取实现的接口
    for (Class clazz : interfaces) {
      System.out.println("interfaceName=" + clazz.getName());
    }
    Constructor[] constructors = studentClass.getConstructors();// 获取所有的构造方法
    for (Constructor constructor : constructors) {
      System.out.println("constructor=" + constructor);
    }
    Annotation[] annotations = studentClass.getAnnotations();// 获取类的注解
    for (Annotation annotation : annotations){
      System.out.println("annotation=" + annotation);
    }
  }
  /**
   * 通过反射的方式获取构造方法，并用构造方法生成对象
   */
  public void testConstructor(){
    Class<Student> studentClass = Student.class;// 获得Class对象
    try {
      Constructor<Student> constructor = (Constructor<Student>)studentClass.getConstructor(String.class,String.class);
      try {
        Student student = constructor.newInstance("1012010638","郑海波");
        student.say();
      } catch (IllegalArgumentException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      } catch (InstantiationException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      } catch (IllegalAccessException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      } catch (InvocationTargetException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }
    } catch (SecurityException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (NoSuchMethodException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }// 获取所有的构造方法
  }

  /**
   * 测试反射方式获取的方法，并调用
   *  输出结果如下;
   *  I am a student,My name is 郑海波,My id is 1012010638
   *  id =1012010638 
   *  I am a student,My name is 郑海波,My id is B08020526
   */
  public void testMethod(){
    Class<Student> studentClass = Student.class;// 获得Class对象
    try {
      Person person = new Student("1012010638","郑海波");
      Method method = studentClass.getMethod("say");
      method.invoke(person, null);//第一个参数是实体对象，第二个参数是该方法的参数
      Method getMethod = studentClass.getMethod("getId");
      String id = (String)getMethod.invoke(person, null);
      System.out.println("id = "+id);
      Method setMethod = studentClass.getMethod("setId",String.class);//第二个(及以后的)参数为方法的参数类型
      setMethod.invoke(person, "B08020526");
      method.invoke(person, null);
    } catch (SecurityException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (NoSuchMethodException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (IllegalArgumentException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (IllegalAccessException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (InvocationTargetException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
  }
  /**
   * 测试获取类的get和set方法
   * @param aClass
   */
  public void printGettersSetters(Class aClass) {
    
    Method[] methods = aClass.getMethods();
    for (Method method : methods) {
      if (isGetter(method))
        System.out.println("getter: " + method);
      if (isSetter(method))
        System.out.println("setter: " + method);
    }
  }

  private boolean isGetter(Method method) {
    if (!method.getName().startsWith("get"))
      return false;
    if (method.getParameterTypes().length != 0)
      return false;
    if (void.class.equals(method.getReturnType()))
      return false;
    return true;
  }

  private boolean isSetter(Method method) {
    if (!method.getName().startsWith("set"))
      return false;
    if (method.getParameterTypes().length != 1)
      return false;
    return true;
  }
  /**
   * 测试访问私有成员变量和私有方法
   */
  public void testPrivateFieldAndMethod(){
    Class<Student> studentClass = null;
    try {
      studentClass = (Class<Student>)Class.forName("edu.njupt.zhb.reflection.Student");
    } catch (ClassNotFoundException e2) {
      // TODO Auto-generated catch block
      e2.printStackTrace();
    }// 获得Class对象
    Student student = new Student("1012010638","郑海波");
    try {
      Field []privateFields = studentClass.getDeclaredFields();//获取私有方法
      for(Field field:privateFields){
        field.setAccessible(true);//设置可访问权限
        String fieldValue = ""+field.get(student);
        String fieldName = field.getName();
        System.out.println(fieldName+":"+fieldValue);
      }
      System.out.println("**获取私有方法----------");
      Method method = studentClass.getDeclaredMethod("privateMethod",null);
      method.setAccessible(true);
      method.invoke(student, null);
    } catch (SecurityException e1) {
      // TODO Auto-generated catch block
      e1.printStackTrace();
    } catch (IllegalArgumentException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (IllegalAccessException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (NoSuchMethodException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (InvocationTargetException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
  }
}

4.输出结果

--基本信息的输出----------------------------------------------------------------
className=edu.njupt.zhb.reflection.Student
field=public boolean edu.njupt.zhb.reflection.Student.isGood
method=public java.lang.String edu.njupt.zhb.reflection.Student.getName()
method=public java.lang.String edu.njupt.zhb.reflection.Student.getId()
method=public void edu.njupt.zhb.reflection.Student.setName(java.lang.String)
method=public void edu.njupt.zhb.reflection.Student.say()
method=public void edu.njupt.zhb.reflection.Student.setId(java.lang.String)
method=public final void java.lang.Object.wait() throws java.lang.InterruptedException
method=public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
method=public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
method=public native int java.lang.Object.hashCode()
method=public final native java.lang.Class java.lang.Object.getClass()
method=public boolean java.lang.Object.equals(java.lang.Object)
method=public java.lang.String java.lang.Object.toString()
method=public final native void java.lang.Object.notify()
method=public final native void java.lang.Object.notifyAll()
false
packagePath=edu.njupt.zhb.reflection
superclassName=java.lang.Object
interfaceName=edu.njupt.zhb.reflection.Person
constructor=public edu.njupt.zhb.reflection.Student()
constructor=public edu.njupt.zhb.reflection.Student(java.lang.String,java.lang.String)
annotation=@java.lang.Deprecated()
--测试构造方法的输出----------------------------------------------------------------
I am a student,My name is 郑海波,My id is 1012010638
--测试invoke方法的输出----------------------------------------------------------------
I am a student,My name is 郑海波,My id is 1012010638
id = 1012010638
I am a student,My name is 郑海波,My id is B08020526
--测试获取get和set方法的输出----------------------------------------------------------------
getter: public java.lang.String edu.njupt.zhb.reflection.Student.getName()
getter: public java.lang.String edu.njupt.zhb.reflection.Student.getId()
setter: public void edu.njupt.zhb.reflection.Student.setName(java.lang.String)
setter: public void edu.njupt.zhb.reflection.Student.setId(java.lang.String)
getter: public final native java.lang.Class java.lang.Object.getClass()
--测试访问私有成员变量和私有函数的输出--------------------------------------------------------
isGood:false
id:1012010638
name:郑海波
**获取私有方法----------
this is a private Method!

下一步将说说Java设计模式之动态代理

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