JVM的String操作

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看看虚拟机是怎么进行字符串操作的

public class Test { /** * @param args */ public static void main(String[] args) { String str = "ab" + "cd"; }}

C:/Java/jdk1.6.0_07/bin>javac Test.java

C:/Java/jdk1.6.0_07/bin>javap -c Test
Compiled from "Test.java"
public class Test extends java.lang.Object{
public Test();
Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
Code:
   0:   ldc     #2; //String abcd
   2:   astore_1
   3:   return

}

从上面可见生成字节码的时候是进行过优化的。

public class Test {
    /**
     * @param args
     */
    public static void main(String[] args) {
    String one = "ab" ;
    String two = "cd " ;
       String str =one + two;
    }
}

C:/Java/jdk1.6.0_07/bin>javac Test.java

public static void main(java.lang.String[]);
Code:
   0:   ldc     #2; //String ab
   2:   astore_1
   3:   ldc     #3; //String cd
   5:   astore_2
   6:   new     #4; //class java/lang/StringBuilder
   9:   dup
   10: invokespecial   #5; //Method java/lang/StringBuilder."<init>":()V
   13: aload_1
   14: invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   17: aload_2
   18: invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   21: invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/la
ng/String;
   24: astore_3
   25: return

}

C:/Java/jdk1.6.0_07/bin>

public class Test {
    /**
     * @param args
     */
    public static void main(String[] args) {
    String one = "ab" ;
    String two = "cd " ;
       String str = one + two + "xx";
    }
}

C:/Java/jdk1.6.0_07/bin>javac Test.java

public static void main(java.lang.String[]);
Code:
   0:   ldc     #2; //String ab
   2:   astore_1
   3:   ldc     #3; //String cd
   5:   astore_2
   6:   new     #4; //class java/lang/StringBuilder
   9:   dup
   10: invokespecial   #5; //Method java/lang/StringBuilder."<init>":()V
   13: aload_1
   14: invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   17: aload_2
   18: invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   21: ldc     #7; //String xx
   23: invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   26: invokevirtual   #8; //Method java/lang/StringBuilder.toString:()Ljava/la
ng/String;
   29: astore_3
   30: return

}

C:/Java/jdk1.6.0_07/bin>