JDK5对字符串+操作进行的优化 Java

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超级潜水艇 写道

第七点已经在JDK6的时候由编译器进行了优化，所以在一般的编写过程中是无需使用StringBuffer或StringBuilder的。编译器会在编译时自动将+的操作转变成StringBuilder的操作。。由我来看使用+操作比使用StringBuilder更加易懂明了（既然编译器已经为你做了优化）。但是有一点要注意。在一个循环中还是得自己写StringBuilder。因为这时编译器是不会给你进行优化的

这个挺好，方便很多。
但没看到官方有说明...

 

这个容易，JDK5编译器就会优化字符串+操作了。不过不要在loop里面使用+(每次loop都会创建一个StringBulider):

 /**
    * @version $Id: StringConcatTest.java Exp $              
   */

public class StringConcatTest {
        //普通字符串相加，JDK5 ↑编译器会自动转为StringBulider处理
        public static String testConcat(String begin){
                return begin+"5"+"2"+"1";
        }
        // 不要循环里相加,每次循环创建一个StringBulider
        public static String testConcatInLoop(String begin){
                for(int i=10;i>0;i--){
                        begin+=""+i;
                }
                return begin;
        }
}

 然后用javap看一下字节码，一目了然：

javap -verbose StringConcatTest > c:\cyy.txt

 下面是文件内容，有点类汇编：

public static java.lang.String testConcat(java.lang.String);
  Code:
   Stack=3, Locals=1, Args_size=1
   0:	new	#16; //class java/lang/StringBuilder
   3:	dup
   4:	aload_0
   5:	invokestatic	#18; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
   8:	invokespecial	#24; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   11:	ldc	#27; //String 5
   13:	invokevirtual	#29; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   16:	ldc	#33; //String 2
   18:	invokevirtual	#29; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   21:	ldc	#35; //String 1
   23:	invokevirtual	#29; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   26:	invokevirtual	#37; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   29:	areturn
  LineNumberTable: 
   line 18: 0

  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      30      0    begin       Ljava/lang/String;


public static java.lang.String testConcatInLoop(java.lang.String);
  Code:
   Stack=3, Locals=2, Args_size=1
   0:	bipush	10
   2:	istore_1
   3:	goto	28
   6:	new	#16; //class java/lang/StringBuilder
   9:	dup
   10:	aload_0
   11:	invokestatic	#18; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
   14:	invokespecial	#24; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   17:	iload_1
   18:	invokevirtual	#44; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
   21:	invokevirtual	#37; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   24:	astore_0
   25:	iinc	1, -1
   28:	iload_1
   29:	ifgt	6  //循环内调用 + 操作，又跳至第6行创建了一个StringBulider
   32:	aload_0
   33:	areturn
  LineNumberTable: 
   line 22: 0
   line 23: 6
   line 22: 25
   line 25: 32

  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      34      0    begin       Ljava/lang/String;
   3      29      1    i       I


}