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