String的Intern方法探析

定义

首先看下String的Intern方法的API定义

 /**     * Returns a canonical representation for the string object.     * <p>     * A pool of strings, initially empty, is maintained privately by the     * class {@code String}.     * <p>     * When the intern method is invoked, if the pool already contains a     * string equal to this {@code String} object as determined by     * the {@link #equals(Object)} method, then the string from the pool is     * returned. Otherwise, this {@code String} object is added to the     * pool and a reference to this {@code String} object is returned.     * <p>     * It follows that for any two strings {@code s} and {@code t},     * {@code s.intern() == t.intern()} is {@code true}     * if and only if {@code s.equals(t)} is {@code true}.     * <p>     * All literal strings and string-valued constant expressions are     * interned. String literals are defined in section 3.10.5 of the     * <cite>The Java&trade; Language Specification</cite>.     *     * @return  a string that has the same contents as this string, but is     *          guaranteed to be from a pool of unique strings.     */    public native String intern();

简单翻译过来就是：
当intern方法被调用时，如果字符串池里已经包含一个相同的字符串对象（使用equals方法判定），那么将返回字符串池中的那个字符串对象，反之，将把这个字符串放入池中，并返回。

可以通过一些简单的例子学习

package com.huyi.jvm;/** * Created with IntelliJ IDEA. * * @Author: huyi * @Date: 2017/11/18 11:04 */public class StringIntern {    public static void main(String[] args) {        String a = new String("ab");        String b = new String("ab");        String c = "ab";        String d = "a" + "b";        String e = "b";        String f = "a" + e;        System.out.println(b.intern() == a);//false        System.out.println(b.intern() == c);//true        System.out.println(b.intern() == d);//true        System.out.println(b.intern() == f);//false        System.out.println(b.intern() == a.intern());//true        System.out.println(f.intern() == a.intern());//true    }}

来分析下这个结果：

1. b.intern() == a ，首先a,b对象都是字符串对象，b.intern()返回的是字符串池的“ab”,"ab" == a 返回的肯定是false2. b.intern() == c ，b.intern()返回的是字符串池的“ab”,"ab" == c,c 也是字符串常量“ab”，这里要说明的是对于字符串常量的比较 "abc" == ("a"+"bc") == ("ab"+"c"),==符号比较的是jvm虚拟机堆中的地址3. b.intern() == d，原理和2相同，都是字符常量的比较4. b.intern() == f，这里为什么是false呢，b.intern()返回的是字符串池的“ab”，而f是字符串对象，两者的地址不一致，所以为false5. b.intern() == a.intern()，字符常量的比较，所以为true6. f.intern() == a.intern()，字符常量的比较，所以为true

所以这里总结下：

1. String.intern()方法返回的是一个字符串常量，这个常量如果已经在字符串池中存在，则直接返回，反之则保存后返回。
2. String的 == 符号比较的是jvm虚拟机堆中的地址

参考：

https://www.cnblogs.com/wanlipeng/archive/2010/10/21/1857513.html
http://blog.csdn.net/chaofanwei/article/details/19486919