StringTable

╰+攻爆jí腚メ 2023-02-09 14:12 15阅读 0赞

## StringTable ##

String s1 = "a";
    String s2 = "b";
    String s3 = "a" + "b";
    String s4 = s1 + s2;
    String s5 = "ab"; 
    String s6 = s4.intern(); 
    
    // 问 
    System.out.println(s3 == s4); 
    System.out.println(s3 == s5); 
    System.out.println(s3 == s6); 
    
    String x2 = new String("c") + new String("d"); 
    String x1 = "cd";
    x2.intern();
    
     // 问，如果调换了【最后两行代码】的位置呢，如果是jdk1.6呢 
     System.out.println(x1 == x2);

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mjg2ODYzOA_size_16_color_FFFFFF_t_70]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mjg2ODYzOA_size_16_color_FFFFFF_t_70 1]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mjg2ODYzOA_size_16_color_FFFFFF_t_70 2]  
超过98%的时间用来做GC并且回收了不到2%的堆内存。用来避免内存过小造成应用不能正常工作。个人感觉，如果真的发生了"GC overhead limit exceeded"错误，那么其实离实际的OOM已经不远了，所以让JVM做个预测提前结束，感觉意义不大。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mjg2ODYzOA_size_16_color_FFFFFF_t_70 3]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mjg2ODYzOA_size_16_color_FFFFFF_t_70 4]

**StringTable 特性**

*  常量池中的字符串仅是符号，第一次用到时才变为对象
 *  利用串池的机制，来避免重复创建字符串对象
 *  字符串变量拼接的原理是 StringBuilder （1.8）
 *  字符串常量拼接的原理是编译期优化
 *  可以使用 intern 方法，主动将串池中还没有的字符串对象放入串池  
    1.8 将这个字符串对象尝试放入串池，如果有则并不会放入，如果没有则放入串池， 会把串 池中的对象返回

**StringTable 性能调优**

*  调整 -XX:StringTableSize=桶个数  
    系统常量字符串比较多的话，需要适当调整此参数
 *  考虑将字符串对象是否入池  
    ![在这里插入图片描述][20200519154206317.png]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mjg2ODYzOA_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/2020051913084792.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mjg2ODYzOA==,size_16,color_FFFFFF,t_70
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[20200519154206317.png]: https://img-blog.csdnimg.cn/20200519154206317.png