JAVA反射使用手记

本篇文章为在工作中使用JAVA反射的经验总结。

      在开始之前，我先定义一个测试类Student，代码如下：

package chb.test.reflect;

public class Student {
    private int age;
    private String name;
    public int getAge() {
        return age;
     }
    public void setAge(int age) {
        this.age = age;
     }
    public String getName() {
        return name;
     }
    public void setName(String name) {
        this.name = name;
     }

    public static void hi(int age,String name){
         System.out.println("大家好，我叫"+name+"，今年"+age+"岁");
     }
}

package chb.test.reflect;

public class Student {
 private int age;
 private String name;
 public int getAge() {
  return age;
 }
 public void setAge(int age) {
  this.age = age;
 }
 public String getName() {
  return name;
 }
 public void setName(String name) {
  this.name = name;
 }
 
 public static void hi(int age,String name){
  System.out.println("大家好，我叫"+name+"，今年"+age+"岁");
 }
}

一、JAVA反射的常规使用步骤

    反射调用一般分为3个步骤：

• 得到要调用类的class
• 得到要调用的类中的方法(Method)
• 方法调用(invoke)

     代码示例：

Class cls = Class.forName("chb.test.reflect.Student");
Method m = cls.getDeclaredMethod("hi",new Class[]{int.class,String.class});
m.invoke(cls.newInstance(),20,"chb");

Class cls = Class.forName("chb.test.reflect.Student");
Method m = cls.getDeclaredMethod("hi",new Class[]{int.class,String.class});
m.invoke(cls.newInstance(),20,"chb");

二、方法调用中的参数类型

        在方法调用中，参数类型必须正确，这里需要注意的是不能使用包装类替换基本类型，比如不能使用Integer.class代替int.class。

       如我要调用Student的setAge方法，下面的调用是正确的：

Class cls = Class.forName("chb.test.reflect.Student");
Method setMethod = cls.getDeclaredMethod("setAge",int.class);
setMethod.invoke(cls.newInstance(), 15);

Class cls = Class.forName("chb.test.reflect.Student");
Method setMethod = cls.getDeclaredMethod("setAge",int.class);
setMethod.invoke(cls.newInstance(), 15);

 

       而如果我们用Integer.class替代int.class就会出错，如：

Class cls = Class.forName("chb.test.reflect.Student");
Method setMethod = cls.getDeclaredMethod("setAge",Integer.class);
setMethod.invoke(cls.newInstance(), 15);

Class cls = Class.forName("chb.test.reflect.Student");
Method setMethod = cls.getDeclaredMethod("setAge",Integer.class);
setMethod.invoke(cls.newInstance(), 15);

 

       jvm会报出如下异常：

java.lang.NoSuchMethodException: chb.test.reflect.Student.setAge(java.lang.Integer)
     at java.lang.Class.getDeclaredMethod(Unknown Source)
     at chb.test.reflect.TestClass.testReflect(TestClass.java:23)

java.lang.NoSuchMethodException: chb.test.reflect.Student.setAge(java.lang.Integer)
 at java.lang.Class.getDeclaredMethod(Unknown Source)
 at chb.test.reflect.TestClass.testReflect(TestClass.java:23)

 

三、static方法的反射调用

 

       static方法调用时，不必得到对象示例，如下：

Class cls = Class.forName("chb.test.reflect.Student");
Method staticMethod = cls.getDeclaredMethod("hi",int.class,String.class);
staticMethod.invoke(cls,20,"chb");//这里不需要newInstance
//staticMethod.invoke(cls.newInstance(),20,"chb");

Class cls = Class.forName("chb.test.reflect.Student");
Method staticMethod = cls.getDeclaredMethod("hi",int.class,String.class);
staticMethod.invoke(cls,20,"chb");//这里不需要newInstance
//staticMethod.invoke(cls.newInstance(),20,"chb"); 

四、private的成员变量赋值

    如果直接通过反射给类的private成员变量赋值，是不允许的，这时我们可以通过setAccessible方法解决。代码示例：

Class cls = Class.forName("chb.test.reflect.Student");
Object student = cls.newInstance();//得到一个实例
Field field = cls.getDeclaredField("age");
field.set(student, 10);
System.out.println(field.get(student));

Class cls = Class.forName("chb.test.reflect.Student");
Object student = cls.newInstance();//得到一个实例
Field field = cls.getDeclaredField("age");
field.set(student, 10);
System.out.println(field.get(student));

 

     运行如上代码，系统会报出如下异常：

java.lang.IllegalAccessException: Class chb.test.reflect.TestClass can not access a member of class chb.test.reflect.Student with modifiers "private"
     at sun.reflect.Reflection.ensureMemberAccess(Unknown Source)
     at java.lang.reflect.Field.doSecurityCheck(Unknown Source)
     at java.lang.reflect.Field.getFieldAccessor(Unknown Source)
     at java.lang.reflect.Field.set(Unknown Source)
     at chb.test.reflect.TestClass.testReflect(TestClass.java:20)

java.lang.IllegalAccessException: Class chb.test.reflect.TestClass can not access a member of class chb.test.reflect.Student with modifiers "private"
 at sun.reflect.Reflection.ensureMemberAccess(Unknown Source)
 at java.lang.reflect.Field.doSecurityCheck(Unknown Source)
 at java.lang.reflect.Field.getFieldAccessor(Unknown Source)
 at java.lang.reflect.Field.set(Unknown Source)
 at chb.test.reflect.TestClass.testReflect(TestClass.java:20)

    解决方法：

Class cls = Class.forName("chb.test.reflect.Student");
Object student = cls.newInstance();
Field field = cls.getDeclaredField("age");
field.setAccessible(true);//设置允许访问
field.set(student, 10);
System.out.println(field.get(student));

Class cls = Class.forName("chb.test.reflect.Student");
Object student = cls.newInstance();
Field field = cls.getDeclaredField("age");
field.setAccessible(true);//设置允许访问
field.set(student, 10);
System.out.println(field.get(student));

    其实，在某些场合下(类中有get,set方法)，可以先反射调用set方法，再反射调用get方法达到如上效果，代码示例：

Class cls = Class.forName("chb.test.reflect.Student");
Object student = cls.newInstance();

Method setMethod = cls.getDeclaredMethod("setAge",Integer.class);
setMethod.invoke(student, 15);//调用set方法

Method getMethod = cls.getDeclaredMethod("getAge");
System.out.println(getMethod.invoke(student));//再调用get方法