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高分求证：StringBuffer和String的性能比较30
先前看到论坛中有人说JDK5下，String的连加操作会被JVM优化为StringBuffer.append()操作，因此拼装String时，两者的性能是一样的。今天闲着没事，写了段小程序测试了一下，对字母A进行1100次拼接，发现StringBuffer的性能比String连加要高出将近100倍，不知是怎么回事。各位给看看，附件是我的测试程序。
问题补充：添加了附件
问题补充：再次添加附件
问题补充：晕，附件添加不进来，直接上程序。public class Test { public static void main(String[] args) { new Test().test(); } public void test() { test1(); test3(); } private void test1() { long t1 = System.nanoTime(); String aa = ""; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; 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aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; aa += "A";aa += "A";aa += "A";aa += "A";aa += "A"; long t2 = System.nanoTime(); System.out.println("连加：" + (t2 - t1)/1000000.0); } private void test3() { long t1 = System.nanoTime(); StringBuffer buffer = new StringBuffer(); for(int i = 0; i < 1100; i++) { buffer.append("A"); } long t2 = System.nanoTime(); System.out.println("buffer：" + (t2 - t1)/1000000.0); } }
问题补充：引用
其实，我更想问的是，为啥之前你不用循环
因为之前看到的帖子上说，JVM不会优化for循环内的String连加操作，所以才会不厌其烦的写了1100个连加。
引用
在字符串修改操作频繁的情况下, 肯定是 StringBuffer 的性能优于 String 的性能.
StringBuffer 出现的目的就是为了改进 String 的缺点
只不过 StringBuffer 是线程安全的, 它会在线程控制方面额外用力, 如果是单线程环境下频繁操作字符串, 可以使用 StringBuilder
这个我知道，我想问的是，之前别的帖子上说，JDK5下JVM会将String连加优化为StringBuilder.append()操作，这样的话，上面的测试中，两者的耗时应该是很接近的，但实际却不是这样，不解。
问题补充：引用
引用
这个我知道，我想问的是，之前别的帖子上说，JDK5下JVM会将String连加优化为StringBuilder.append()操作，这样的话，上面的测试中，两者的耗时应该是很接近的，但实际却不是这样，不解。
他这么说有给出他的依据么? 这个可以咨询他一下的.
但是据我查看 JDK 5 的 changenote, 是没有提到 String 的这一优化方式的.
http://java.sun.com/j2se/1.5.0/ReleaseNotes.html#intro
这个是原帖，倒数第三楼。
http://www.iteye.com/topic/585121
引用为啥你那么纠结性能?既然你知道StringBuffer 和 String 在处理不同问题上的优点.为啥你还要去关注性能呢?不解!
如果说JDK5下两者的性能一样的话，那以后编程过程中就不用刻意使用StringBuffer了，String连加不是更方便吗？这才是我要验证这个说法的最终目的。
引用很明显这位大神在浪费时间。。。
哎，世风日下！
问题补充：引用
如果官方没有文档的话,
1) 可以咨询这个讨论主题里提出这个观点的人
2) 搜索相关主题
因为在这里, 很难遇到对这个在官方文档都没有提及的问题有深入了解的人
这两步现在都做了，原帖中的当事人还没有回复。
这个优化在官方文档是否有说明，不太清楚。但在网上一搜，很多帖子都谈到了这个问题，应该不是以讹传讹吧，呵呵。
刚才又看到一篇帖子，貌似解释的不错，可以参照下：
http://cache.baidu.com/c?m=9f65cb4a8c8507ed4fece7631046893b4c4380147d8c8c4668d4e419ce3b4c413037bfa6663f405a8e906b6075a81e0fb9f367263c0123b59999cf1d8dac925f75ce786a6459db0144dc41eb945125b061d61ab2f34cf0ba8173d5ee898f8a04149e07523a&p=b47ac800c5d912a058bd9b7d0d48&user=baidu
可能我的程序中不应该使用+=号。
问题补充：引用
你在看一下,别的帖子说的String连加会不会是
aa="A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"
而不是
aa += "A";aa += "A";aa += "A";aa += "A";aa += "A";
呵呵，好像确实是这样。第二种会多创建出很多小对象。
引用
我记得当时我侧过10w循环的测试
String 和StringBuffer 时间没有任何差别！！
我不知道你是怎么测得啊
如上，估计是测试程序的问题。2010年7月07日 20:59
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采纳的答案
JVM优化是对的，我这里没有JDK5.0.用的是6.0测试的，6.0对于String的连加采用的是StringBuilder优化的，按说优化了为什么StringBuffer的性能还是那么好呢。下面就分析一下。测试用例如下：
/** * @author 飞雪无情 * @since 2010-7-10 */ public class TestStringBuffer { public void testString(){ String s=""; for(int i=0;i<10000;i++){ s+=i; } } public void testStringBuffer(){ StringBuffer s=new StringBuffer(""); for(int i=0;i<10000;i++){ s.append(i); } } }
这两个函数分别演示了string的连加和StringBuffer操作，执行测试，我本机上testString()600多毫秒,testStringBuffer()不超过20毫秒。下面就分析为什么，我们采用JDK的反汇编工具javap,执行javap -c TestStringBuffer后我们看到CMD输出,我们挑重要的看，先看方法testString。public void testString(); Code: 0: ldc #19; //String 2: astore_1 3: iconst_0 4: istore_2 5: goto 30 8: new #21; //class java/lang/StringBuilder 11: dup 12: aload_1 13: invokestatic #23; //Method java/lang/String.valueOf:(Ljava/lang/Objec t;)Ljava/lang/String; 16: invokespecial #29; //Method java/lang/StringBuilder."<init>":(Ljava/la ng/String;)V 19: iload_2 20: invokevirtual #32; //Method java/lang/StringBuilder.append:(I)Ljava/la ng/StringBuilder; 23: invokevirtual #36; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 26: astore_1 27: iinc 2, 1 30: iload_2 31: sipush 10000 34: if_icmplt 8 37: return
我就不一行行分析了，有兴趣的可以看看相关知识，分析关键的，循环从“2:”开始到“34:”看第“8:”，创建了一个StringBuilder，然后使用append方法("第20:")进行String连加的优化,最要注意的是 创建这个StringBuilder，是在循环里面，也就是说我们循环多少次就会创建多少个StringBuilder，这就很影响性能(创建对象是很耗性能的).然后我们看testStringBuffer方法。public void testStringBuffer(); Code: 0: new #46; //class java/lang/StringBuffer 3: dup 4: ldc #19; //String 6: invokespecial #48; //Method java/lang/StringBuffer."<init>":(Ljava/lan g/String;)V 9: astore_1 10: iconst_0 11: istore_2 12: goto 24 15: aload_1 16: iload_2 17: invokevirtual #49; //Method java/lang/StringBuffer.append:(I)Ljava/lan g/StringBuffer; 20: pop 21: iinc 2, 1 24: iload_2 25: sipush 10000 28: if_icmplt 15 31: return
看看多么简单，在循环开始前创建好StringBuffer，然后再循环里面调用append方法(第17:),只创建了一个对象，然后只在缓冲区里面进行字符操作。2010年7月10日 13:02
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你在看一下,别的帖子说的String连加会不会是
aa="A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"+"A"
而不是
aa += "A";aa += "A";aa += "A";aa += "A";aa += "A";2010年7月08日 21:05
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如果官方没有文档的话,
1) 可以咨询这个讨论主题里提出这个观点的人
2) 搜索相关主题
因为在这里, 很难遇到对这个在官方文档都没有提及的问题有深入了解的人2010年7月08日 14:13
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引用这个我知道，我想问的是，之前别的帖子上说，JDK5下JVM会将String连加优化为StringBuilder.append()操作，这样的话，上面的测试中，两者的耗时应该是很接近的，但实际却不是这样，不解。
他这么说有给出他的依据么? 这个可以咨询他一下的.
但是据我查看 JDK 5 的 changenote, 是没有提到 String 的这一优化方式的.
http://java.sun.com/j2se/1.5.0/ReleaseNotes.html#intro2010年7月08日 12:14
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为啥你那么纠结性能?既然你知道StringBuffer 和 String 在处理不同问题上的优点.为啥你还要去关注性能呢?不解!
2010年7月08日 10:49
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在字符串修改操作频繁的情况下, 肯定是 StringBuffer 的性能优于 String 的性能.
StringBuffer 出现的目的就是为了改进 String 的缺点
只不过 StringBuffer 是线程安全的, 它会在线程控制方面额外用力, 如果是单线程环境下频繁操作字符串, 可以使用 StringBuilder2010年7月07日 23:16
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