Intern method of string

String.intern() It's a local (native) Method , Its function is if the string constant pool already contains an equal to this String Object's string , Returns the String References to objects , Otherwise, a String Object to the string constant pool , And return a reference to this object , stay jdk6 Or before, the constant pool in the virtual machine is allocated to the permanent generation , We can go through －xx:PermSize and -xx:MaxPermSize Limit the size of the permanent generation , You can indirectly limit the capacity of its constant pool , stay jdk8 Metaspace is completely used in place of permanent generation .
Sample code ：

public static void main(String[] args) {
		String str=new StringBuilder("58").append("tongcheng").toString();
		System.out.println(str);
		System.out.println(str.intern());
		System.out.println(str==str.intern());
		System.out.println();

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

        String s="java";
		System.out.println(s==str2.intern());
	}

Execution results ：

58tongcheng
58tongcheng
true

java
java
false
true

Why? 58tongcheng yes true and java Of str2==str2.intern() yes false？
Because there is an initialized java character string (jdk Self contained ) In the load sum.misc.Version When this class enters the constant pool

 sun.misc.Version Class will be in JDK The class library is loaded and initialized during initialization , During initialization, it needs to set the static constant field according to the specified constant value (ConstantValue） Do default initialization , At this time sun.misc.Version.launcher Static constant field refers to "java" The string literal is intern To HotSpot VM String constant pool ——StringTable In the .  
therefore   new StringBuilder("ja").append("va").toString(); When is new One. , Print the first line str2 It was this when new String , The second sentence str2.intern() yes jdk Bring this with you launcher_name Variable java value , So print out false. Not the same String object

String s="java";
System.out.println(s==str2.intern()); by true,s It's the original jdk Loaded java character string