Java中字符串String的+和+=及循环操作String原理详解

String对象是不可变的：意思就是无论是对String的新增或修改，出现一个全新的String内容时，都意味着诞生了一个新的对象。但是如果内容不变的话，增加的只是对象的引用而已。

例如：

String a = "ljh";
String b = "ljh";
String c = "ljh";

System.out.println(a==b);
System.out.println(b==c);

结果都是true

但是这种不可变性会产生一些性能上的问题，所以JVM对String对象重载“+”“+=”进行了一些优化

操作符“+”可以用来连接String

String aaa = "ljh";
String bbb = "big";
String ccc = aaa+bbb+"aaaa";

在jdk8中，上述代码中在底层其实是编译器擅自调用了StringBuilder类进行+的操作，主要原因是StringBuilder的append（）更加高效，我们来看一下字节码。

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String ljh
       2: astore_1
       3: ldc           #3                  // String big
       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 aaaa
      23: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      26: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      29: astore_3
      30: return

可以看出一共有四个对象，分别是三个String 和一个StringBuilder

我们再来看一下+=

String a = "aaa";
a += "bbb";

字节码如下

Code:

0: ldc #2 // String aaa

2: astore_1

3: new #3 // class java/lang/StringBuilder

6: dup

7: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V

10: aload_1

11: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

14: ldc #6 // String bbb

16: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

19: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

22: astore_1

23: return

可以看出先创建了一个“aaa”字符串，然后当a遇到+=时，创建了一个StringBuilder对象，并append了aaa字符串。之后创建了一个“bbb”对象，然后append了bbb字符串，最后调用StringBuilder的toString方法。

接下来再看看循环中调用+=会是什么样子

String a = "aaa";
a += "bbb";
for(int i=0;i<5;i++){
    a+="ccc";
}

Code:

0: ldc #2 // String aaa

2: astore_1

3: new #3 // class java/lang/StringBuilder

6: dup

7: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V

10: aload_1

11: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

14: ldc #6 // String bbb

16: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

19: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

22: astore_1

23: iconst_0

24: istore_2

25: iload_2

26: iconst_5

27: if_icmpge 56

30: new #3 // class java/lang/StringBuilder

33: dup

34: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V

37: aload_1

38: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

41: ldc #8 // String ccc

43: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

46: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

49: astore_1

50: iinc 2, 1

53: goto 25

56: return

可以看出先创建String对象aaa,之后创建StringBuilder并初始化StringBuilder append aaa，然后创建bbb对象，并append（ bbb），然后我们发现在循环中依旧创建了一个新的StringBuilder，也就是没经过一次循环都要创建一个新的StringBuilder对象。

这时我们做一个优化，提前创建StringBuilder对象

String a = "aaa";
a += "bbb";
StringBuilder sb = new StringBuilder(a);
for(int i=0;i<5;i++){
    sb.append("ccc");
}

Code:

0: ldc #2 // String aaa

2: astore_1

3: new #3 // class java/lang/StringBuilder

6: dup

7: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V

10: aload_1

11: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

14: ldc #6 // String bbb

16: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

19: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

22: astore_1

23: new #3 // class java/lang/StringBuilder

26: dup

27: aload_1

28: invokespecial #8 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V

31: astore_2

32: iconst_0

33: istore_3

34: iload_3

35: iconst_5

36: if_icmpge 52

39: aload_2

40: ldc #9 // String ccc

42: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

45: pop

46: iinc 3, 1

49: goto 34

52: return

可以看出循环体跳回34行，并不会不断地创建新的StringBuilder，大大提高了效率和减小了垃圾数量！，所以我们要注意自己的写法！避免无谓的消耗

总结

原文地址：https://blog.csdn.net/qq_31615049/article/details/80891142