Java字符串优化

Question

这会创建多少个字符串？

String test(String text) {
  return "string 1 " +
    text + " string 2 " +
    "string 3";
}

Answer 1

这里不存在新的 String 调用，唯一存在的调用位于 StringBuilder.toString 中，因此

1

java.lang.String test(java.lang.String);
  Code:
   Stack=3, Locals=2, Args_size=2
   0:   new #16; //class java/lang/StringBuilder
   3:   dup
   4:   ldc #18; //String string 1 
   6:   invokespecial   #20; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   9:   aload_1
   10:  invokevirtual   #23; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   13:  ldc #27; //String  string 2 
   15:  invokevirtual   #23; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   18:  ldc #29; //String string 3
   20:  invokevirtual   #23; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   23:  invokevirtual   #31; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   26:  areturn

Answer 2

通过编译代码，然后使用 javap -c 检查字节码，可以很容易地回答这个问题。在这种特殊情况下，编译器应该生成这样的代码，

String result = new StringBuilder("string1").
    append(text).append("string2").append("string3").toString();

具体取决于您如何看待它，您可能会说只有一个，或者如果您是那些喜欢在使用文字时将其视为“创建”的人之一，你可以说四个。

Answer 3

javap是你的朋友。我将您的代码放入 S.java 中，并使用 javap -c S 对其进行反汇编：

Compiled from "S.java"
public class S extends java.lang.Object{
public S();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public java.lang.String test(java.lang.String);
  Code:
   0:   new     #2; //class java/lang/StringBuilder
   3:   dup
   4:   invokespecial   #3; //Method java/lang/StringBuilder."<init>":()V
   7:   ldc     #4; //String string 1 
   9:   invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   12:  aload_1
   13:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   16:  ldc     #6; //String  string 2 
   18:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   21:  ldc     #7; //String string 3
   23:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   26:  invokevirtual   #8; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   29:  areturn

}

在 OpenJDK 1.6.0_22 中，仅创建了一个 StringBuilder 和一个 String。

Answer 4

这相当于

 (new StringBuilder((new StringBuilder((new StringBuilder("String 1")).append(text).toString())).append("string 2 ").toString()).append("string 3").toString();

3 个字符串生成器和 6 个字符串。优化的方法是：

    StringBuilder sb = new StringBuilder("string 1");
    sb.append(test);
    sb.append(" string 2 ");
    sb.append("string 3");
    return sb.toString();