深入理解java虚拟机（三）：String.intern()-字符串常量池

看源码：

 public native String intern();
 
 Returns a canonical representation for the string object.

A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.

All literal strings and string-valued constant expressions are interned. String literals are defined in §3.10.5 of the Java Language Specification

Returns:
a string that has the same contents as this string, but is guaranteed to be from a pool of unique strings.

 

意思是说当调用 intern 方法时，如果池已经包含一个等于此 String 对象的字符串（该对象由 equals(Object) 方法确定），则返回池中的字符串。否则，将此 String 对象添加到池中，并且返回此 String 对象的引用。

看下面这段代码：

public class StringInternTest {

    public static void main(String[] args) {

        String str1 = new StringBuilder("chaofan").append("wei").toString();
        System.out.println(str1.intern() == str1);

        String str2 = new StringBuilder("ja").append("va").toString();
        System.out.println(str2.intern() == str2);
    }
}

有图可知，当在jdk1.6中运行时，会得到两个false，而在jdk1.7中运行，会得到一个true和一个false。
产生的差异在于在jdk1.6中 intern 方法会把首次遇到的字符串实例复制到永久待（常量池）中，并返回此引用；但在jdk1.7中，只是会把首次遇到的字符串实例的引用添加到常量池中（没有复制），并返回此引用。
所以在jdk1.7中执行上面代码，str1返回true是引用他们指向的都是str1对象（堆中）（池中不存在，返回原引用），而str2返回false是因为池中已经存在"java"了（关键词），所以返回的池的对象，因此不相等。

 

看下面实例：

@Test
    public void test3(){

        String str1 = "a";
        String str2 = "b";
        String str3 = "ab";
        String str4 = str1 + str2;
        String str5 = new String("ab");

        System.out.println(str5.equals(str3));//true
        System.out.println(str5 == str3);//false
        System.out.println(str5.intern() == str3);//true
        System.out.println(str5.intern() == str4);//false


    }
    @Test
    public void test4(){

        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);//fasle
        System.out.println(b.intern() == c);//true
        System.out.println(b.intern() == d);//true  编译期d已确定(修改、赋值)为ab
        System.out.println(b.intern() == f);//false
        System.out.println(b.intern() == a.intern());//true


    }
    @Test
    public void test5(){
        // 编译期已确定
        String a = "abc";
        String b = "abc";
        String c = "a" + "b" + "c";
        String d = "a" + "bc";
        String e = "ab" + "c";

        System.out.println(a == b);//true
        System.out.println(a == c);//true
        System.out.println(a == d);//true
        System.out.println(a == e);//true
        System.out.println(c == d);//true
        System.out.println(c == e);//true


    }