本篇文章給大家分享的是有關(guān)如何理解java reflect，小編覺得挺實(shí)用的，因此分享給大家學(xué)習(xí)，希望大家閱讀完這篇文章后可以有所收獲，話不多說，跟著小編一起來看看吧。

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            SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();

            Class selectorImplClass =
                    Class.forName("sun.nio.ch.SelectorImpl", false, ClassLoader.getSystemClassLoader());

            // Ensure the current selector implementation is what we can instrument.
            if (!selectorImplClass.isAssignableFrom(selector.getClass())) {
                return selector;
            }

            Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
            Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");

            selectedKeysField.setAccessible(true);
            publicSelectedKeysField.setAccessible(true);

            selectedKeysField.set(selector, selectedKeySet);
            publicSelectedKeysField.set(selector, selectedKeySet);

            selectedKeys = selectedKeySet;

先看第一行， SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet(); 這個(gè)東西干嗎用的，替代selector默認(rèn)實(shí)現(xiàn)當(dāng)中的selectKeys這個(gè)字段，當(dāng)中的實(shí)現(xiàn)非常牛逼。后續(xù)再看。

跳過無關(guān)緊要的部分，直接看這段

Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");

取得2個(gè)field，目的是忘對象selector里面設(shè)值。

            selectedKeysField.setAccessible(true);
            publicSelectedKeysField.setAccessible(true);

            selectedKeysField.set(selector, selectedKeySet);
            publicSelectedKeysField.set(selector, selectedKeySet);

然后就是設(shè)值可訪問屬性，并且修改selector的2個(gè)屬性（selectedKeys 和 publicSelectedKeys ），設(shè)值為之前new出來的 selectedKeySet ，再讓 selectedKeys = selectedKeySet ，selector在執(zhí)行select的時(shí)候，就會修改 selectedKeys 當(dāng)中的屬性值。

然后再看看 SelectedSelectionKeySet 當(dāng)中究竟干了些什么：

final class SelectedSelectionKeySet extends AbstractSet {

    private SelectionKey[] keysA;
    private int keysASize;
    private SelectionKey[] keysB;
    private int keysBSize;
    private boolean isA = true;

    SelectedSelectionKeySet() {
        keysA = new SelectionKey[1024];
        keysB = keysA.clone();
    }

    @Override
    public boolean add(SelectionKey o) {
        if (o == null) {
            return false;
        }

        if (isA) {
            int size = keysASize;
            keysA[size ++] = o;
            keysASize = size;
            if (size == keysA.length) {
                doubleCapacityA();
            }
        } else {
            int size = keysBSize;
            keysB[size ++] = o;
            keysBSize = size;
            if (size == keysB.length) {
                doubleCapacityB();
            }
        }

        return true;
    }

    private void doubleCapacityA() {
        SelectionKey[] newKeysA = new SelectionKey[keysA.length << 1];
        System.arraycopy(keysA, 0, newKeysA, 0, keysASize);
        keysA = newKeysA;
    }

    private void doubleCapacityB() {
        SelectionKey[] newKeysB = new SelectionKey[keysB.length << 1];
        System.arraycopy(keysB, 0, newKeysB, 0, keysBSize);
        keysB = newKeysB;
    }

    SelectionKey[] flip() {
        if (isA) {
            isA = false;
            keysA[keysASize] = null;
            keysBSize = 0;
            return keysA;
        } else {
            isA = true;
            keysB[keysBSize] = null;
            keysASize = 0;
            return keysB;
        }
    }

    @Override
    public int size() {
        if (isA) {
            return keysASize;
        } else {
            return keysBSize;
        }
    }

    @Override
    public boolean remove(Object o) {
        return false;
    }

    @Override
    public boolean contains(Object o) {
        return false;
    }

    @Override
    public Iterator iterator() {
        throw new UnsupportedOperationException();
    }
}

用 keysA 和 keysB 2個(gè)數(shù)組，輪流保存SelectionKey，非常方便的實(shí)現(xiàn)了讀寫分離，用數(shù)組代替了set，極大地提高了內(nèi)存訪問效率。

以上就是如何理解java reflect，小編相信有部分知識點(diǎn)可能是我們?nèi)粘９ぷ鲿姷交蛴玫降摹ＯＭ隳芡ㄟ^這篇文章學(xué)到更多知識。更多詳情敬請關(guān)注創(chuàng)新互聯(lián)行業(yè)資訊頻道。