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: ()Vflags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1   // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 1: 0public java.lang.String incStringJoint(); // incStringJoint()方法执行情况descriptor: ()Ljava/lang/String;flags: ACC_PUBLICCode:stack=2, locals=3, args_size=10: ldc           #2   // String a2: astore_13: new           #3   // class java/lang/StringBuilder6: dup7: invokespecial #4   // Method java/lang/StringBuilder."<init>":()V10: aload_111: invokevirtual #5   // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;14: ldc           #6   // String b16: 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_223: aload_224: areturnLineNumberTable:      //反编译结果对应源码的行数line 4: 0line 5: 3line 6: 23public java.lang.String incStringBuffer(); // incStringBuffer()方法执行情况descriptor: ()Ljava/lang/String;flags: ACC_PUBLICCode:stack=3, locals=2, args_size=10: new           #8    // class java/lang/StringBuffer3: dup4: iconst_35: invokespecial #9    // Method java/lang/StringBuffer."<init>":(I)V8: astore_19: aload_110: ldc           #2    // String a12: invokevirtual #10   // Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;15: pop16: aload_117: ldc           #6    // String b19: invokevirtual #10   // Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;22: pop23: aload_124: invokevirtual #11   // Method java/lang/StringBuffer.toString:()Ljava/lang/String;27: areturnLineNumberTable:      //反编译结果对应源码的行数line 10: 0line 11: 9line 12: 16line 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_PUBLICCode:stack=2, locals=4, args_size=10: ldc           #2                  // String a2: astore_13: new           #3                  // class java/lang/StringBuilder6: dup7: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V10: aload_111: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;14: ldc           #6                  // String b16: 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_223: new           #3                  // class java/lang/StringBuilder26: dup27: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V30: aload_231: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;34: ldc           #8                  // String c36: 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_343: aload_344: areturnLineNumberTable:line 4: 0line 5: 3line 6: 23line 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: ()Vflags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1                  // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 1: 0public java.lang.String incString();descriptor: ()Ljava/lang/String;flags: ACC_PUBLICCode:stack=1, locals=2, args_size=10: ldc           #2                  // String a2: astore_13: aload_14: areturnLineNumberTable:line 20: 0line 21: 3public java.lang.String incNewString();descriptor: ()Ljava/lang/String;flags: ACC_PUBLICCode:stack=3, locals=2, args_size=10: new           #3                  // class java/lang/String3: dup4: ldc           #2                  // String a6: invokespecial #4                  // Method java/lang/String."<init>":(Ljava/lang/String;)V9: astore_110: aload_111: areturnLineNumberTable:line 25: 0line 26: 10
}

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