Java中关于StringBuilder和+号字符串拼接的效率问题

有这么一个例子：

String a = "nihao";
String b = "1233";
int c = 21;
String res = a+b+"dd"+c;

开始我就是这样写的，然后同时看到了跟我说最好用string builder，效率高一点，然后我感觉有道理，就改了：

StringBuilder builder = new StringBuilder();
builder.append("nihao").append("1233").append(21);

但是这个时候ide提醒我如下，就是让我该回去

Reports any usages of StringBuffer, StringBuilder or StringJoiner which can be replaced with a single java.lang.String concatenation. Using a String concatenation makes the code shorter and simpler. This inspection only reports when the resulting concatenation is at least as efficient or more efficient than the original code.

网上重新查了查，我就重新写了个测试，验证一下不同。
test1：

public void test1(){
        StringBuilder builder = new StringBuilder();
        builder.append("nihao").append("1233").append(21);
        System.out.println(builder.toString());
    }

编译后：

public void test1() {
        StringBuilder builder = new StringBuilder();
        builder.append("nihao").append("1233").append(21);
        System.out.println(builder.toString());
    }

用builder没有什么不同。
test2：

public void test2(){
        String a = "nihao";
        String b = "1233";
        int c = 21;
        String res = a+b+"dd"+c;
        String two = "dffe"+123+"ddd";
    }

编译后：

public void test2() {
        String a = "nihao";
        String b = "1233";
        int c = 21;
        (new StringBuilder()).append(a).append(b).append("dd").append(c).toString();
        String two = "dffe123ddd";
    }

这个时候不同就多了。看一下反编译结果（javap -c TestDemo.class）：

  public void test1();
    Code:
       0: new           #2                  // class java/lang/StringBuilder
       3: dup
       4: invokespecial #3                  // Method java/lang/StringBuilder."":()V
       7: astore_1
       8: aload_1
       9: ldc           #4                  // String nihao
      11: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      14: ldc           #6                  // String 1233
      16: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      19: bipush        21
      21: invokevirtual #7                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      24: pop
      25: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
      28: aload_1
      29: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      32: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      35: return

  public void test2();
    Code:
       0: ldc           #4                  // String nihao
       2: astore_1
       3: ldc           #6                  // String 1233
       5: astore_2
       6: bipush        21
       8: istore_3
       9: new           #2                  // class java/lang/StringBuilder
      12: dup
      13: invokespecial #3                  // Method java/lang/StringBuilder."":()V
      16: aload_1
      17: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      20: aload_2
      21: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: ldc           #11                 // String dd
      26: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      29: iload_3
      30: invokevirtual #7                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      33: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      36: astore        4
      38: ldc           #12                 // String dffe123ddd
      40: astore        5
      42: return

最后，再结合网上的其他测试，有以下几点：
1、如果+拼接的时候没有对象，比如String two = "dffe"+123+"ddd";，这种情况效率最高，不要画蛇添足的非要用string builder。你看编译后的结果，JVM直接就做了拼接转化，根本不用builder.
2、如果你要拼接的包含对象，比如String res = a+b+"dd"+c;，而且不用于循环，这个时候用+ 和 用builder 随意，都一样。
3、一行 + 操作就会产生一个builder（当然，分号结尾才是一行）。比如String res = a+b+"dd"+c; res += "555";会new 两次builder。这和用在循环一个道理，同样每次循环都会重新new一个builder。所以用在循环或者多行操作的时候，builder是效率最高的，次数少的话和buffer是差不多的。