深入理解java虛擬機（三）：String.intern()-字符串常量池

看源碼：

 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.

 

意思是說當調用 intern 方法時，如果池已經包含一個等於此 String 對象的字符串（該對象由 equals(Object) 方法確定），則返回池中的字符串。否則，將此 String 對象添加到池中，並且返回此 String 對象的引用。

看下面這段代碼：

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);
    }
}

有圖可知，當在jdk1.6中運行時，會得到兩個false，而在jdk1.7中運行，會得到一個true和一個false。
產生的差異在於在jdk1.6中 intern 方法會把首次遇到的字符串實例複製到永久待（常量池）中，並返回此引用；但在jdk1.7中，只是會把首次遇到的字符串實例的引用添加到常量池中（沒有複製），並返回此引用。
所以在jdk1.7中執行上面代碼，str1返回true是引用他們指向的都是str1對象（堆中）（池中不存在，返回原引用），而str2返回false是因爲池中已經存在"java"了（關鍵詞），所以返回的池的對象，因此不相等。

 

看下面實例：

@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  編譯期d已確定(修改、賦值)爲ab
        System.out.println(b.intern() == f);//false
        System.out.println(b.intern() == a.intern());//true


    }
    @Test
    public void test5(){
        // 編譯期已確定
        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


    }