String "+" 的补充说明---行粒度

String 中“+” 的操作的补充说明

在使用“+”的时候，会创建一个StringBuilder对象，然后invokevirtual append()操作

“+”操作创建StringBuilder的粒度是行代码。

String str = a+b+c;

这就是一行，这样他就会只new 一个StringBuider对象然后执行append操作，最后在调用toString()操作。

下面是两个例子：

public class Main{

    public static void main(String[]args){

        String a ="a",b="b",c="c";

        String d = a+b+c;

    }

}

这段简单代码的源码

  public static void main(java.lang.String[]);

    flags: ACC_PUBLIC, ACC_STATIC

    Code:

      stack=2, locals=5, args_size=1

         0: ldc           #2                  // String a

         2: astore_1      

         3: ldc           #3                  // String b

         5: astore_2      

         6: ldc           #4                  // String c

         8: astore_3      

         9: new           #5                  // class java/lang/StringBuilder

        12: dup           

        13: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V

        16: aload_1       

        17: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

        20: aload_2       

        21: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

        24: aload_3       

        25: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

        28: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

        31: astore        4

        33: return        

      LineNumberTable:

        line 3: 0

        line 4: 9

        line 5: 33

在上面字节码文件中可以看见只有一个new（第九行）,然后就执行了相关append 操作。-----------行粒度，这里是一行。、

 

下面设置为两行：

 

1 public class Main{

2     public static void main(String[]args){

3         String a ="a",b="b",c="c";

4         String d = a+b;

5             d = d+c; //切为两行

6     }

7 }

 

在次看看字节码

public static void main(java.lang.String[]);

    flags: ACC_PUBLIC, ACC_STATIC

    Code:

      stack=2, locals=5, args_size=1

         0: ldc           #2                  // String a

         2: astore_1      

         3: ldc           #3                  // String b

         5: astore_2      

         6: ldc           #4                  // String c

         8: astore_3      

         9: new           #5                  // class java/lang/StringBuilder

        12: dup           

        13: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V

        16: aload_1       

        17: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

        20: aload_2       

        21: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

        24: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

        27: astore        4

        29: new           #5                  // class java/lang/StringBuilder

        32: dup           

        33: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V

        36: aload         4

        38: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

        41: aload_3       

        42: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

        45: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

        48: astore        4

        50: return        

      LineNumberTable:

        line 3: 0

        line 4: 9

        line 5: 29

        line 6: 50

 

在 9 、29两行 new