疯狂的java反射机制

int32位 posted @ Feb 19, 2014 09:54:57 AM in java , 4934 阅读
转载请注明:http://krystism.is-programmer.com/若有错误,请多多指正,谢谢!

java 从1.1就开始支持反射机制了,c++不支持反射机制。通过反射机制能够获取对象的类型和这个对象能做什么(有什么属性,什么方法),能够动态调用对象的方法,设置属性,以及创建新的实例(因此除了利用new关键字创建实例,还可以通过反射机制创建实例,其实还有两种方法,即反序列化和clone)。利用java的反射机制可以实现一些疯狂的事,做一些貌似无法实现的事。本文主要参照了2009年JavaZone会议上Heinz Kabutz发表的关于反射技术的疯狂演说 ,写得不错!

我们知道java的private属性是不允许外部直接访问修改的,而只能通过调用对象的外部接口访问和修改,但这不是绝对的,利用反射机制可以不调用对象的公共方法而修改对象属性,甚至修改final属性的值。

java String中字符存储是使用了private final char[] value,从这里我们也可以看出为什么java 字符串是不允许修改的,因为value是final的,但这仅意味着你无法通过调用它的公有方法来改变它的值,利用反射机制可以绕过这些限制而修改它,如代码:(想想这段代码的输出会是什么呢?)

import java.lang.reflect.*;
public class Test {
	public static void main(String[] args) throws Exception {
		Field field = String.class.getDeclaredField("value");
		field.setAccessible(true);
		field.set("hello!", "cheers".toCharArray());
		System.out.println("hello!");
	}
}

这里还有一个问题是字符串池,当使用多个字面字符串时(不使用new String("...")),如果已经存在这个字符串,则不会创建一个新的对象,而仅仅指向已存在字符串的引用,即

String s1 = "hello";
String s2 = "hello";
System.out.println(s1 == s2);

s1 和s2是同一个对象的引用,即s1 == s2返回true。

回到上一段代码,field.set("hello!", "cheers".toCharArray()) 和 System.out.println("hello!")中字符串"hello!"其实是同一个对象,利用反射机制改变这个对象的value值,因此此时字符串"hello!"对应的value其实已经变成了"cheers",因此上段代码输出为cheers而不是hello!。

还有更神奇的,猜猜下面这段代码的输出结果是什么?

import java.lang.reflect.*;
public class Test {
	public static void main(String[] args) throws Exception {
		Field field = null;
		field = Integer.class.getDeclaredField("value");
		field.setAccessible(true);
		field.set(42, 43);
		System.out.printf("6 * 7 = %d\n", 6 * 7);
	}
}

利用反射机制还可以做很多奇妙的事,比如获取方法调用者,获取类中的方法,获取对象的内存大小等等,具体参照原文!

转载请注明:http://krystism.is-programmer.com/若有错误,请多多指正,谢谢!
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olddoor 说:
2015年4月29日 15:13

写的很不错.浅显易懂.

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Emma 说:
2023年1月18日 19:06

Java reflection is a powerful tool that allows developers to inspect and manipulate the internals of Java classes and objects at runtime. However, it can also be used to cbd benefits create some pretty crazy things. For example, there is a Java library that allows you to change the value of a variable without using assignment statements!

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