Java 字节码之解析1

Java 字节码之解析一

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例子：一个Java类 TestString.java

Compiled from "TestString.java"public class TestString extends java.lang.Object{public TestString();  Code:   0:      aload_0   1:      invokespecial  #8; //Method java/lang/Object."<init>":()V   4:      return public java.lang.String testString(java.lang.String, java.lang.String);  Code:   0:      new #16; //class java/lang/StringBuilder   3:      dup   4:      aload_1   5:      invokestatic    #18; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;   8:      invokespecial  #24; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V   11:     aload_2   12:    invokevirtual  #27; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;   15:    invokevirtual  #31; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;   18:    areturn public java.lang.String testStringBuffer(java.lang.StringBuffer, java.lang.String);  Code:   0:      aload_1   1:      aload_2   2:      invokevirtual  #40; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;   5:      invokevirtual  #45; //Method java/lang/StringBuffer.toString:()Ljava/lang/String;   8:      areturn } 

从上面编译后的字节码信息可以看出来，方法testString 调用了五个方法：new 、invokestatic、invokespecial 和两个invokevirtual ; 而testStringBuffer 方法只调用了两个invokevirtual 方法。第一个方法比第二个方法多做了好多工作，其效率当然是要低的。而且我们从java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

可以看出来其实对于String字符串合并，内部还是转化为StringBuilder的方法调用，这是因为String是长度不可变的，所以不如直接采用StringBuilder（与StringBuffer 长度都是可变的，只不过前者是非线程安全，后者是线程安全）进行字符串合并。
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