编译器是否为常量字符串优化了Stringbuffer/StringBuilder.append（）？

Question

我读Jon Skeet's answer about concatenating strings with +。我想知道编译器是否也识别用StringBuffer/StringBuilder附加constand字符串。

这对于构建URL代码具有良好的意愿：

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，如果的StringBuffer.append（）不是由编译器为常数优化，我会说下面的代码会更有效：

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

为什么不试试呢？在Java 1.7，下面的类的主要方法：

 
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()); 
    } 
} 

（I改变的恒定为了清楚的数目）得到下面的字节代码：

 
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   
} 

而下面的类：

 
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()); 
    } 
} 

被编译为：

 
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。在第二类中，StringBuilder的追加方法只被调用两次，而在第一种情况下，它被调用每个常量字符串。

Answer 2

编译器是否为常量字符串优化了Stringbuffer/StringBuilder.append（）？

号

不过，我认为你提出的优化的前提是不正确。我建议你采用这两个版本的代码并编译它们。然后使用javap来查看编译代码在每种情况下的样子。 （FWIW，我希望你的“优化”不会改进生成的代码，它很可能会创建第二个StringBuilder来连接中间字符串，并将其转换为字符串，最后得到的数字大致相同append操作，外加一个额外的临时StringBuilder和一个额外的临时String的创建。）