Java本地vs实例变量访问速度

发布时间：2020-12-15 07:38:09 所属栏目：Java 来源：网络整理

导读：所以我的问题是关于 Java中的变量访问速度.今天在我的“CS”(如果你可以称之为)中,老师提出了类似下面的列表示例： public class ListExampleT { private NodeT head; private NodeT tail; private class NodeT { /* ... */ } public void append(T content)

所以我的问题是关于 Java中的变量访问速度.今天在我的“CS”(如果你可以称之为)中,老师提出了类似下面的列表示例：

public class ListExample<T> {
    private Node<T> head;
    private Node<T> tail;

    private class Node<T> { /* ... */ }

    public void append(T content) {
        if (!isEmpty()) {
            Node<T> dummy = new Node<T>(content);
            head = dummy;
            tail = dummy;

            head.setNext(head);
            // or this
            dummy.setNext(dummy);

        } else { /* ... */ }
    }

    // more methods
    // ...
}

我的问题是：对head.setNext(head)的调用是否比dummy.setNext(dummy)慢？即使它不明显.
我想知道这一点,因为显然头部和类的实例变量和虚拟是本地的,所以本地访问会更快吗？

解决方法

好的,我已经编写了一个微基准测试程序(由@Joni& @MattBall建议),以下是每个本地变量和一个实例变量的1 x 1000000000次访问的结果：

Average time for instance variable access: 5.08E-4
Average time for local variable access: 4.96E-4

每次10 x 1000000000次访问：

Average time for instance variable access:4.723E-4
Average time for local variable access:4.631E-4

对于100 x 1000000000访问每个：

Average time for instance variable access: 5.050300000000002E-4
Average time for local variable access: 5.002400000000001E-4

所以看起来局部变量访问确实比实例var访问更快(即使两者都指向同一个对象).

注意：我不想找到这个,因为我想要优化的东西,这只是纯粹的兴趣.

附：以下是微基准的代码：

public class AccessBenchmark {
    private final long N = 1000000000;
    private static final int M = 1;

    private LocalClass instanceVar;

    private class LocalClass {
        public void someFunc() {}
    }

    public double testInstanceVar() {
        // System.out.println("Running instance variable benchmark:");
        instanceVar = new LocalClass();

        long start = System.currentTimeMillis();
        for (int i = 0; i < N; i++) {
            instanceVar.someFunc();
        }

        long elapsed = System.currentTimeMillis() - start;

        double avg = (elapsed * 1000.0) / N;

        // System.out.println("elapsed time = " + elapsed + "ms");
        // System.out.println(avg + " microseconds per execution");

        return avg;
    }

    public double testLocalVar() {
        // System.out.println("Running local variable benchmark:");
        instanceVar = new LocalClass();
        LocalClass localVar = instanceVar;

        long start = System.currentTimeMillis();
        for (int i = 0 ; i < N; i++) {
            localVar.someFunc();
        }

        long elapsed = System.currentTimeMillis() - start;

        double avg = (elapsed * 1000.0) / N;

        // System.out.println("elapsed time = " + elapsed + "ms");
        // System.out.println(avg + " microseconds per execution");

        return avg;
    }

    public static void main(String[] args) {
        AccessBenchmark bench;

        double[] avgInstance = new double[M];
        double[] avgLocal = new double[M];

        for (int i = 0; i < M; i++) {
            bench = new AccessBenchmark();

            avgInstance[i] = bench.testInstanceVar();
            avgLocal[i] = bench.testLocalVar();

            System.gc();
        }

        double sumInstance = 0.0;
        for (double d : avgInstance) sumInstance += d;
        System.out.println("Average time for instance variable access: " + sumInstance / M);

        double sumLocal = 0.0;
        for (double d : avgLocal) sumLocal += d;
        System.out.println("Average time for local variable access: " + sumLocal / M);
    }
}

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!