所以我的问题是关于 Java 中的可变访问速度。今天在我的“CS”(如果你可以这样调用它)中，老师给出了一个与以下列表类似的例子:

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:调用 head.setNext(head) 会比 dummy.setNext(dummy) 慢吗？即使它不明显。 我想知道这一点，因为 head 显然是类的实例 var 和 dummy 是本地的，所以本地访问会更快吗？

最佳答案

好的，我已经编写了一个微型基准测试(正如@Joni 和@MattBall 所建议的那样)，这里是对每个局部变量和实例变量进行 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

对于每个 10 x 1000000000 次访问:

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

对于每个 100 x 1000000000 次访问:

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

因此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:我并不想发现这个，因为我想优化一些东西，这只是纯粹的兴趣。

附言这是微基准测试的代码:

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);
    }
}

关于Java 本地 vs 实例变量访问速度，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/21613098/