JAVA虚拟机——利用javap反编译class文件分析代码执行过程

我们经常会看到或者听到java代码要如何如何优化，这样写不对，那样写性能不高等等，可是我们如何去伪求真呢？就以 Java 代码性能优化总结文章中的内容例子，来求证一下吧

先热热身

String字符串拼接和StringBuffer一样吗

看看下面这段代码：

public class TestClass{
    
    public String incStringJoint(){
        String str = "a";
        String str1 = str + "b";
        return str1;
    }
    
    public String incStringBuffer(){
        StringBuffer str = new StringBuffer(3);
        str.append("a");
        str.append("b");
        return str.toString();  
    }
    
}

抛开其他方面，这两段代码性能一样吗？ 带着这个问题，我们去探索一番。

javac TestClass.java    //将java文件编译成class文件
javap -verbose TestClass.class  //反编译class文件

反编译结果：

{
  public TestClass();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1   // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0
        
  
  public java.lang.String incStringJoint(); // incStringJoint()方法执行情况
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: ldc           #2   // String a
         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 b
        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_2
        23: aload_2
        24: areturn
      LineNumberTable:      //反编译结果对应源码的行数
        line 4: 0
        line 5: 3
        line 6: 23
    
  public java.lang.String incStringBuffer(); // incStringBuffer()方法执行情况
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=2, args_size=1
         0: new           #8    // class java/lang/StringBuffer
         3: dup
         4: iconst_3
         5: invokespecial #9    // Method java/lang/StringBuffer."<init>":(I)V
         8: astore_1
         9: aload_1
        10: ldc           #2    // String a
        12: invokevirtual #10   // Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
        15: pop
        16: aload_1
        17: ldc           #6    // String b
        19: invokevirtual #10   // Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
        22: pop
        23: aload_1
        24: invokevirtual #11   // Method java/lang/StringBuffer.toString:()Ljava/lang/String;
        27: areturn
      LineNumberTable:      //反编译结果对应源码的行数
        line 10: 0
        line 11: 9
        line 12: 16
        line 13: 23
}
SourceFile: "TestClass.java"

我们先大致看一下反编译的结果，可以发现其实String字符串拼接实质上是用StringBuilder的append方法实现，那么我们再看一下StringBuffer和StringBuilder的源码：可以发现，两个类的append方法调用的是同一个方法实现字符串拼接。

    public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

这里就有一个问题了，那为什么在字符串拼接的时候，要使用StringBuffer（线程安全）或者使用StringBuilder（线程不安全）呢？

现在我们修改一下代码：

    public String incStringJoint(){
        String str = "a";
        String str1 = str + "b";
        String str2 = str1 + "c";
        return str2;
    }

多次拼接，看看结果怎么？

public java.lang.String incStringJoint();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=4, args_size=1
         0: ldc           #2                  // String a
         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 b
        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_2
        23: new           #3                  // class java/lang/StringBuilder
        26: dup
        27: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
        30: aload_2
        31: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        34: ldc           #8                  // String c
        36: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        39: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        42: astore_3
        43: aload_3
        44: areturn
      LineNumberTable:
        line 4: 0
        line 5: 3
        line 6: 23
        line 7: 43

注意第3行和第23行，new了两个StringBuilder。这就说明，利用String本身进行字符串拼接的时候，每拼接一次，就要new一个StringBuilder，再调用append方法。

new String()到底创建了几个对象

看下面代码：

public class TestClass{
    
    public String incString(){
        String str = "a";
        return str;
    }
    
    public String incNewString(){
        String str = new String("a");
        return str; 
    }

}



{
  public TestClass();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0

  public java.lang.String incString();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=2, args_size=1
         0: ldc           #2                  // String a
         2: astore_1
         3: aload_1
         4: areturn
      LineNumberTable:
        line 20: 0
        line 21: 3

  public java.lang.String incNewString();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=2, args_size=1
         0: new           #3                  // class java/lang/String
         3: dup
         4: ldc           #2                  // String a
         6: invokespecial #4                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
         9: astore_1
        10: aload_1
        11: areturn
      LineNumberTable:
        line 25: 0
        line 26: 10
}

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