java不可变类慢得多

所以我创建了简单测试速度可变对vs不可变：

final class MutableInt {
    private int value;

    public int getValue() {
        return value;
    }

    public void setValue(int value) {
        this.value = value;
    }

    public MutableInt() {
        this(0);
    }

    public MutableInt(int value) {
        this.value = value;
    }   
}

final class ImmutableInt {
    private final int value;

    public ImmutableInt(int value) {
        this.value = value;
    }

    public int getValue() {
        return value;
    }
}

public class TestImmutableSpeed {

    static long testMutable(final int arrLen) {
        MutableInt[] arrMutable = new MutableInt[arrLen];
        for (int i = 0; i < arrMutable.length; ++i) {
            arrMutable[i] = new MutableInt(i);
            for (int j = 0; j < arrMutable.length; ++j) {
                arrMutable[i].setValue(arrMutable[i].getValue() + j);
            }
        }
        long sumMutable = 0;
        for (MutableInt item : arrMutable) {
            sumMutable += item.getValue();
        }
        return sumMutable;
    }

    static long testImmutable(final int arrLen) {
        ImmutableInt[] arrImmutable = new ImmutableInt[arrLen];
        for (int i = 0; i < arrImmutable.length; ++i) {
            arrImmutable[i] = new ImmutableInt(i);
            for (int j = 0; j < arrImmutable.length; ++j) {
                arrImmutable[i] = new ImmutableInt(arrImmutable[i].getValue() + j);
            }
        }
        long sumImmutable = 0;
        for (ImmutableInt item : arrImmutable) {
            sumImmutable += item.getValue();
        }
        return sumImmutable;
    }

    public static void main(String[] args) {
        final int arrLen = 1<<14;

        long tmStart = System.nanoTime();
        System.out.println("sum = " + testMutable(arrLen));
        long tmMid = System.nanoTime();
        System.out.println("sum = " + testImmutable(arrLen));
        long tmEnd = System.nanoTime();

        System.out.println("speed comparison mutable vs immutable:");
        System.out.println("mutable   " + (tmMid - tmStart)/1000000 + " ms");
        System.out.println("immutable " + (tmEnd - tmMid)/1000000 + " ms");
    }
}

如果测试运行速度太慢/快速,您可以调整阵列的大小.

我运行：-server -Xms256m -XX：AggressiveOpts
我得到：

sum = 2199023247360
sum = 2199023247360
speed comparison mutable vs immutable:
mutable   102 ms
immutable 1506 ms

问题：我缺少一些优化参数,还是不可变的版本15x慢？

如果是这样,为什么有人用不可变的类复杂写数学库？是不可变的只是“花哨”但没用吗？

我知道不可变类是哈希映射密钥更安全,或者不具有竞争条件,但是这些特殊情况可以在任何地方都可以无变化地处理.

编辑：我按照一个答案建议,用卡钳重新运行这个微型基准测试,它运行速度是12倍,而不是15倍.更改Caliper基准测试代码：

import com.google.caliper.Runner;
import com.google.caliper.SimpleBenchmark;



final class MutableInt {
    private int value;

    public int getValue() {
        return value;
    }

    public void setValue(int value) {
        this.value = value;
    }

    public MutableInt() {
        this(0);
    }

    public MutableInt(int value) {
        this.value = value;
    }   
}

final class ImmutableInt {
    private final int value;

    public ImmutableInt(int value) {
        this.value = value;
    }

    public int getValue() {
        return value;
    }
}


public class TestImmutableSpeed extends SimpleBenchmark {

    static long testMutable(final int arrLen) {
        MutableInt[] arrMutable = new MutableInt[arrLen];
        for (int i = 0; i 

卡尺输出：

 0% Scenario{vm=java,trial=0,benchmark=Mutable,type=-server,minMemory=-Xms256m,optimizations=-XX:+AggressiveOpts} 91614044.60 ns; ?=250338.20 ns @ 3 trials
50% Scenario{vm=java,benchmark=Immutable,optimizations=-XX:+AggressiveOpts} 1108057922.00 ns; ?=3920760.98 ns @ 3 trials

benchmark     ms linear runtime
  Mutable   91.6 ==
Immutable 1108.1 ==============================

请注意,没有Caliper的JVM输出的优化参数是：

 0% Scenario{vm=java,benchmark=Mutable} 516562214.00 ns; ?=623120.57 ns @ 3 trials
50% Scenario{vm=java,benchmark=Immutable} 1706758503.00 ns; ?=5842389.60 ns @ 3 trials

benchmark   ms linear runtime
  Mutable  517 =========
Immutable 1707 ==============================

所以糟糕的参数使得这两个版本都比较慢,但比例不太可怕(但并不重要).