public class StringPool58Demo {
    public static void main(String[] args) {
        String str1 = new StringBuilder("58").append("tongcheng").toString();
        System.out.println(str1); // 58tongcheng
        System.out.println(str1.intern()); // 58tongcheng
        System.out.println(str1 == str1.intern()); //true
        System.out.println("------------");
        String str2 = new StringBuilder("ja").append("va").toString();
        System.out.println(str2); // java
        System.out.println(str2.intern());  // java
        System.out.println(str2 == str2.intern()); // false ⚠️ 有一个初始化的java字符串(JDK出娘胎自带的)， 在加载sun.misc.Version这个类的时候进入常量池
    }
}

public final class System {

    /* register the natives via the static initializer.
     *
     * VM will invoke the initializeSystemClass method to complete
     * the initialization for this class separated from clinit.
     * Note that to use properties set by the VM, see the constraints
     * described in the ⚠️initializeSystemClass method.
     */
     ...
    /**
     * Initialize the system class.  Called after thread initialization. 在线程初始化后被调用
     */
    private static void initializeSystemClass() {
       ...
     	 sun.misc.Version.init();
     	 ...
     }
}

// sun.misc.Version.init()方法所在类
package sun.misc;

import java.io.PrintStream;

public class Version {
    private static final String launcher_name = "java"; ⚠️
    private static final String java_version = "1.8.0_261";
    private static final String java_runtime_name = "Java(TM) SE Runtime Environment";
    private static final String java_profile_name = "";
    private static final String java_runtime_version = "1.8.0_261-b12";
}