Question

我读了Jon Skeet's answer about concatenating strings with +。我想知道编译器是否还使用StringBuffer / StringBuilder识别附加constand字符串。

构建URL的代码有一个很好的意图：

StringBuffer sb = new StringBuffer(constant1);
sb.append(nonconstant);
sb.append("?");
sb.append(constant2);
sb.append("=");
sb.append(constant3);
sb.append("&");
sb.append(constant4);
sb.append("=");
sb.append(constant5);

但是，如果编译器没有为常量优化Stringbuffer.append（），我会说以下代码会更有效：

StringBuffer sb = new StringBuffer(constant1);
sb.append(non-constant);
sb.append("?" + constant2 + "=" + constant3 + "&" + constant4 + "=" + constant5);

因为编译器会在编译时优化+字符串连接。

Answer 1

为什么不尝试一下？在java 1.7中，以下类的主要方法是：

public class Concat1
{
    private static final String constant2 = "c2";
    private static final String constant3 = "c3";

    public void main(String[] args) 
    {
        StringBuilder sb = new StringBuilder();
        sb.append(args[0]);
        sb.append("?");
        sb.append(constant2);
        sb.append("=");
        sb.append(constant3);
        System.out.println(sb.toString());
    }
}

（为了清晰起见，我更改了常量的数量）产生以下字节代码：

public class Concat1 {
  public Concat1();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."":()V
       4: return        

  public void main(java.lang.String[]);
    Code:
       0: new           #2                  // class java/lang/StringBuilder
       3: dup           
       4: invokespecial #3                  // Method java/lang/StringBuilder."":()V
       7: astore_2      
       8: aload_2       
       9: aload_1       
      10: iconst_0      
      11: aaload        
      12: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      15: pop           
      16: aload_2       
      17: ldc           #5                  // String ?
      19: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      22: pop           
      23: aload_2       
      24: ldc           #6                  // String c2
      26: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      29: pop           
      30: aload_2       
      31: ldc           #7                  // String =
      33: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      36: pop           
      37: aload_2       
      38: ldc           #8                  // String c3
      40: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      43: pop           
      44: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
      47: aload_2       
      48: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      51: invokevirtual #11                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      54: return        
}

以下课程：

public class Concat2
{
    private static final String constant2 = "c2";
    private static final String constant3 = "c3";

    public void main(String[] args) 
    {
        StringBuilder sb = new StringBuilder();
        sb.append(args[0]);
        sb.append("?" + constant2 + "=" + constant3);
        System.out.println(sb.toString());
    }
}

编译为：

public class Concat2 {
  public Concat2();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."":()V
       4: return        

  public void main(java.lang.String[]);
    Code:
       0: new           #2                  // class java/lang/StringBuilder
       3: dup           
       4: invokespecial #3                  // Method java/lang/StringBuilder."":()V
       7: astore_2      
       8: aload_2       
       9: aload_1       
      10: iconst_0      
      11: aaload        
      12: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      15: pop           
      16: aload_2       
      17: ldc           #5                  // String ?c2=c3
      19: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      22: pop           
      23: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
      26: aload_2       
      27: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      30: invokevirtual #8                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      33: return        
}

显然，你是对的。在第二个类中，StringBuilder的append方法只被调用两次，而在第一个例子中，它被调用每个常量字符串。

Answer 2

编译器是否针对常量字符串优化Stringbuffer / StringBuilder.append（）？

没有

但是，我认为您提出的优化的前提是不正确的。我建议您使用两个版本的代码并编译它们。然后使用javap查看编译代码在每种情况下的样子。

（FWIW，我希望您的＆＃34;优化＆＃34;不会改进生成的代码。很可能会创建第二个StringBuilder来连接中间字符串，并将其转换为String。最终得到大约相同数量的append操作，再加上创建额外的临时StringBuilder和额外的临时String。）

编译器是否针对常量字符串优化Stringbuffer / StringBuilder.append（）？

2 个答案: