JAVA6可以使用字符串累加

<< 我肯定没有这样说过…这帖里说的东西都跟HotSpot（或者其它JVM实现）一点关系都没有，还在Java源码级编译器层次上…


<< 这个也不对…要分情况说。前面也有说了，String的+本来就是用StringBuilder或StringBuffer实现的。
关键问题是：是不是同一个StringBuilder/StringBuffer。在循环里用+=之类的方式来拼接字符串的问题就出在每轮循环里都new了一个StringBuilder/StringBuffer来做拼接，然后toString()完就抛弃了，等下轮循环进来又再new一个。


嗯，从Java字节码一层能看到的情况看，确实是如此的。

让我演示一下某代码在JDK 1.0.2上编译出来的样子：

public class X {
  public static void main(String[] args) {
    String a = "alpha";
    String b = "beta";
    String c = "charlie";
    String d = a + b + c;
  }
}

Compiled from "X.java"
public class X extends java.lang.Object
  SourceFile: "X.java"
  minor version: 3
  major version: 45
  Constant pool:
const #1 = String       #16;    //  alpha
const #2 = String       #8;     //  beta
const #3 = String       #22;    //  charlie
const #4 = class        #17;    //  java/lang/Object
const #5 = class        #10;    //  X
const #6 = Method       #4.#7;  //  java/lang/Object."<init>":()V
const #7 = NameAndType  #20:#23;//  "<init>":()V
const #8 = Asciz        beta;
const #9 = Asciz        ConstantValue;
const #10 = Asciz       X;
const #11 = Asciz       Exceptions;
const #12 = Asciz       LineNumberTable;
const #13 = Asciz       SourceFile;
const #14 = Asciz       LocalVariables;
const #15 = Asciz       Code;
const #16 = Asciz       alpha;
const #17 = Asciz       java/lang/Object;
const #18 = Asciz       main;
const #19 = Asciz       ([Ljava/lang/String;)V;
const #20 = Asciz       <init>;
const #21 = Asciz       X.java;
const #22 = Asciz       charlie;
const #23 = Asciz       ()V;

{
public static void main(java.lang.String[]);
  Code:
   Stack=1, Locals=4, Args_size=1
   0:   ldc     #1; //String alpha
   2:   astore_1
   3:   ldc     #2; //String beta
   5:   astore_2
   6:   ldc     #3; //String charlie
   8:   astore_3
   9:   return
  LineNumberTable:
   line 3: 0
   line 4: 3
   line 5: 6
   line 2: 9


public X();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #6; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 1: 0


}

<< 留意一下Class文件的版本号，这个是用JDK 1.0.2里的javac编译的，如假包换。
看到了么，main()方法里的字节码根本就没有字符串拼接的动作发生，对吧？

然后用JDK 1.0.2的javac -O来编译：

Compiled from "X.java"
public class X extends java.lang.Object
  SourceFile: "X.java"
  minor version: 3
  major version: 45
  Constant pool:
const #1 = class        #11;    //  java/lang/Object
const #2 = class        #6;     //  X
const #3 = Method       #1.#4;  //  java/lang/Object."<init>":()V
const #4 = NameAndType  #14:#16;//  "<init>":()V
const #5 = Asciz        ConstantValue;
const #6 = Asciz        X;
const #7 = Asciz        Exceptions;
const #8 = Asciz        SourceFile;
const #9 = Asciz        LocalVariables;
const #10 = Asciz       Code;
const #11 = Asciz       java/lang/Object;
const #12 = Asciz       main;
const #13 = Asciz       ([Ljava/lang/String;)V;
const #14 = Asciz       <init>;
const #15 = Asciz       X.java;
const #16 = Asciz       ()V;

{
public static void main(java.lang.String[]);
  Code:
   Stack=0, Locals=1, Args_size=1
   0:   return

public X();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #3; //Method java/lang/Object."<init>":()V
   4:   return

}

加了-O参数之后main()方法里啥也不剩了，只有个return。
这才是Java层的“优化”…

然则从Sun的JDK 1.3开始javac就忽略-O参数了。现在大家用JDK6里的javac就看不到这种效果。

================================================

换个例子，

public class Y {
  public static void main(String[] args) {
    String a = "alpha";
    String b = "beta";
    String c = "charlie";
    String d = a + b + c;
    System.out.println(d);
  }
}

还是用JDK 1.0.2里的javac，不带-O参数来编译：

Compiled from "Y.java"
public class Y extends java.lang.Object
  SourceFile: "Y.java"
  minor version: 3
  major version: 45
  Constant pool:
const #1 = String       #32;    //  alpha
const #2 = String       #22;    //  beta
const #3 = String       #45;    //  charlie
const #4 = class        #35;    //  java/lang/StringBuffer
const #5 = class        #36;    //  java/lang/Object
const #6 = class        #25;    //  java/io/PrintStream
const #7 = class        #24;    //  Y
const #8 = class        #42;    //  java/lang/System
const #9 = Method       #5.#18; //  java/lang/Object."<init>":()V
const #10 = Method      #4.#17; //  java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
const #11 = Field       #8.#15; //  java/lang/System.out:Ljava/io/PrintStream;
const #12 = Method      #4.#18; //  java/lang/StringBuffer."<init>":()V
const #13 = Method      #6.#16; //  java/io/PrintStream.println:(Ljava/lang/String;)V
const #14 = Method      #4.#19; //  java/lang/StringBuffer.toString:()Ljava/lang/String;
const #15 = NameAndType #33:#41;//  out:Ljava/io/PrintStream;
const #16 = NameAndType #20:#34;//  println:(Ljava/lang/String;)V
const #17 = NameAndType #44:#39;//  append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
const #18 = NameAndType #40:#46;//  "<init>":()V
const #19 = NameAndType #31:#21;//  toString:()Ljava/lang/String;
const #20 = Asciz       println;
const #21 = Asciz       ()Ljava/lang/String;;
const #22 = Asciz       beta;
const #23 = Asciz       ConstantValue;
const #24 = Asciz       Y;
const #25 = Asciz       java/io/PrintStream;
const #26 = Asciz       Exceptions;
const #27 = Asciz       LineNumberTable;
const #28 = Asciz       SourceFile;
const #29 = Asciz       LocalVariables;
const #30 = Asciz       Code;
const #31 = Asciz       toString;
const #32 = Asciz       alpha;
const #33 = Asciz       out;
const #34 = Asciz       (Ljava/lang/String;)V;
const #35 = Asciz       java/lang/StringBuffer;
const #36 = Asciz       java/lang/Object;
const #37 = Asciz       main;
const #38 = Asciz       ([Ljava/lang/String;)V;
const #39 = Asciz       (Ljava/lang/String;)Ljava/lang/StringBuffer;;
const #40 = Asciz       <init>;
const #41 = Asciz       Ljava/io/PrintStream;;
const #42 = Asciz       java/lang/System;
const #43 = Asciz       Y.java;
const #44 = Asciz       append;
const #45 = Asciz       charlie;
const #46 = Asciz       ()V;

{
public static void main(java.lang.String[]);
  Code:
   Stack=2, Locals=5, Args_size=1
   0:   ldc     #1; //String alpha
   2:   astore_1
   3:   ldc     #2; //String beta
   5:   astore_2
   6:   ldc     #3; //String charlie
   8:   astore_3
   9:   new     #4; //class java/lang/StringBuffer
   12:  dup
   13:  invokespecial   #12; //Method java/lang/StringBuffer."<init>":()V
   16:  aload_1
   17:  invokevirtual   #10; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
   20:  aload_2
   21:  invokevirtual   #10; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
   24:  aload_3
   25:  invokevirtual   #10; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
   28:  invokevirtual   #14; //Method java/lang/StringBuffer.toString:()Ljava/lang/String;
   31:  astore  4
   33:  getstatic       #11; //Field java/lang/System.out:Ljava/io/PrintStream;
   36:  aload   4
   38:  invokevirtual   #13; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   41:  return
  LineNumberTable:
   line 3: 0
   line 4: 3
   line 5: 6
   line 6: 9
   line 7: 33
   line 2: 41


public Y();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #9; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 1: 0


}

当时的Java就已经是用StringBuffer来实现字符串拼接（+）的了。
这也毫无出奇之处，在Java语言规范里就有这么写：

顺带一提现在在JDK6里用的是Java语言规范第三版，而JDK7即将使用的是Java语言规范第四版（还没出）。

================================================

JVM的实现内部也有一些针对字符串拼接的优化的。但那些看Java字节码是看不出来的——在这个抽象层之下，屏蔽了“优化”这种细节。

例如说JRockit里有特制的jrockit.vm.StringMaker类专门用来优化字符串拼接，JRockit的JIT编译器会识别出StringBuilder的使用模式，发现符合条件的时候就把一些StringBuilder“悄悄的”转换为这种特别的StringMaker类来处理。

大家用得比较多的Oracle/Sun的HotSpot VM的比较新的版本里也有类似的优化，可以把符合条件的一些相邻的StringBuilder合并为一个，用于优化这种场景：

String a = x + y + z;
String b = a + x + y;

（假定变量a在后面就没有再被使用过了）
本来这段代码会被javac编译为等价于下面的代码：

String a = new StringBuilder().append(x).append(y).append(z).toString();
String b = new StringBuilder().append(a).append(x).append(y).toString();

而被HotSpot的JIT编译器优化后，会变成类似下面的样子：

String b = new StringBuilder().append(x).append(y).append(z).append(x).append(y).toString();

也就是把相邻的StringBuilder合并掉了。再次留意这个例子的前提是变量a在后面就没有被用过了，只有变量b在后面还有被用到。
这才是“JVM做的优化”。字节码层面就能看出来的所谓“优化”根本还没到JVM那层。

 

	public static void main(String[] args) {

		StringBuffer sb = new StringBuffer();
		long l3 = System.currentTimeMillis();
		for (int i = 0; i < 10000; i++) {
			sb.append("a").append(i);
		}
		long l4 = System.currentTimeMillis();
		System.out.println(l4 - l3);
		System.out.println(sb.length());

		String s = "";
		long l = System.currentTimeMillis();
		for (int i = 0; i < 10000; i++) {
			s = s + "a" + i;
		}
		long l2 = System.currentTimeMillis();
		System.out.println(l2 - l);
		System.out.println(s.length());

	}

 

结果：

stringbuffer：

 

0

48890

 

String：

1281

48890

 

jdk1.6

 

根本就没优化过啊啊，或者说优化之后也是这个鸟样！坑爹呐

不要写任何依赖语法糖 依赖编译器的代码
久而久之 你就总会以为有虚拟机给你优化
这是不好的习惯

这个讨论意义不大
代码不应该依赖于jdk版本或者JVM实现
否则可移植性太差了

 

两位有点过于追求完美了，你不会从jdk1.6切换会jdk1.4以下吧。

我指导得最多的就是不要在简单的sql拼接时使用过多stringbuild，比如：
  execute("select * from " + tableName + " where id=?", 1);

有人过度强调原则，如前两位，结果写了
  execute(
    new StringBuilder("select * from ").append(tableName).append(" where id=?"), 1);

如果给人看还是直接+更加容易读

所以需要权衡，我的一般原则：
  1. 循环中操作字符串，使用StringBuilder
  2. 如果字符串特别长，同时还不能一次使用+完成所有操作 时使用StringBuilder

这个只是个人经验。

 

请仔细看贴。“在循环中不要使用累加操作”