这两个Java段的性能有何不同？

Question

我很想知道这两个Java方法调用中的任何一个在处理器时间,内存分配和/或垃圾收集方面是否会表现得完全不同.

SomeObject myObj = new SomeObject();
myObj.doSomething();

与

new SomeObject().doSomething();

Answer 1

查看生成的字节码:

// code 1
new SomeObject().doSomething();

// bytecode 1
   0:   new #2; //class SomeObject
   3:   dup
   4:   invokespecial   #3; //Method SomeObject."<init>":()V
   7:   invokevirtual   #4; //Method SomeObject.doSomething:()V
   10:  return

你可以清楚地看到这个有另外两个指令:

// code 2
SomeObject myObj = new SomeObject();
myObj.doSomething();

// bytecode 2
   0:   new #2; //class SomeObject
   3:   dup
   4:   invokespecial   #3; //Method SomeObject."<init>":()V
   7:   astore_1
   8:   aload_1
   9:   invokevirtual   #4; //Method SomeObject.doSomething:()V
   12:  return

这些指令似乎非常冗余且易于优化.我敢打赌JIT编译器会在需要时处理它们.