Java:在双重检查惯用语中使用局部变量 [英] Java: using a local variable in double check idiom

问题描述

Josh Bloch提到在双重检查惯用语中使用局部变量以提高性能(EJ，第二版，第284页)，并说示例代码在其机器上的运行速度比没有局部变量的计算机快25％.所以问题是，为什么速度更快?对于局部变量版本，它将访问实例变量3次，在正常情况下，它将访问实例变量4次.是"25％"增长背后的原因还是其他原因?

Josh Bloch mentions using a local variable in double check idiom for possible performance improvement (EJ, 2nd ed. p284) and says the example code runs about 25% faster on his machine compared to that without a local variable. So the question is, why is it faster? In case of local variable version, it accesses the instance variable 3 times and in normal case it accesses the instance variable 4 times. Is this the reason behind '25%' increase or are there any other reasons?

只有在首次创建实例时，才进行3或4次访问.此后是1或2次.

3 or 4 times access is only when the instance is first created. Thereafter, it's 1 or 2 times.

检查该问题的可接受答案以查看示例代码.我认为这是针对Java 6的.如何解决双重检查锁定被破坏"的问题，请参见解决方案".用Java声明吗?

check the accepted answer for this question to see the example code. I think this is for Java 6. How to solve the "Double-Checked Locking is Broken" Declaration in Java?

推荐答案

最基本的是，访问 volatile 变量要比访问本地变量慢.声明局部变量时，基本上是在方法中缓存易失变量的值.

The basic thing is that accessing the volatile variable is slower, than accessing the local one. When you declare a local variable, you are basically caching the value of a volatile variable inside the method.

在正常情况下(没有局部变量)，您将访问volatile变量:

In the normal (without a local variable) case you are accessing the volatile variable:

1. 在 synchronized 子句之前的第一个 if 中
2. 在 synchronized 子句中的第二个 if 中
3. 在第二个 if 中，为它分配值
4. 在 return 语句中

1. In the first if before the synchronized clause
2. In the second if inside the synchronized clause
3. Inside the second if where you assign a value to it
4. In the return statement

现在，如果您引入局部变量，则只需访问 volatile 变量三次:

Now, if you introduce a local variable, you only access the volatile variable three times:

1. 在为 synchronized 子句之前的第一个 if 分配局部变量时
2. 在 synchronized 子句中为第二个 if 分配局部变量时
3. 在第二个 if 中，为它分配值

1. When assigning the local variable for the first if before the synchronized clause
2. When assigning the local variable for the second if inside the synchronized clause
3. Inside the second if where you assign a value to it

通过返回局部变量，您不会在 return 语句中访问它，从而提高了速度.

You do not access it in the return statement, by returning the local variable, thus giving you a speed boost.

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