Java

引用分类

• 强引用-StrongReference：引用指向对象，gc(Garbage Collection)运行时不回收。
• 软引用-SoftReference：运行时可能回收(jvm内存不够)。常做缓存。
• 弱引用-WeakReference：gc运行时立即回收。
• 虚引用-PhantomReference：类似于无引用，主要跟踪对象被回收的状态，不能单独使用，必须与引用队列(ReferenceQueue)联合使用。

    public void test1() {
        // 字符串常量池 共享(不能回收)
        String str = "You are very good!";

        // 弱引用管理对象
        WeakReference<String> wr = new WeakReference<String>(str);

        Log.d(TAG,"gc运行前：" + wr.get());

        // 断开引用
        str = null;

        // 通知回收
        System.gc();

        // 开始回收
        System.runFinalization();

        Log.d(TAG,"gc运行后：" + wr.get());
    }

gc运行前：You are very good!
gc运行后：You are very good!

public void test2() {
        
    String str = new String("You are very good!");

    // 弱引用管理对象
    WeakReference<String> wr = new WeakReference<String>(str);

    Log.d(TAG,"gc运行前：" + wr.get());

    // 断开引用
    str = null;

    // 通知回收
    System.gc();

    // 开始回收
    System.runFinalization();

    Log.d(TAG,"gc运行后：" + wr.get());
}
gc运行前：You are very good!
gc运行后：null

1. WeakHashMap 键为弱引用，回收键后自动删除key-value对象

public void test3() {

    WeakHashMap<String,String> map = new WeakHashMap<>();

    // 测试数据
    // 常量池对象，不会回收
    map.put("a","a");
    map.put("b","b");

    // gc运行 已被回收
    map.put(new String("c"),"c");
    map.put(new String("d"),"d");

    System.gc();
    System.runFinalization();

    int a  = map.size();

}

map.size() = 3,包含 a b d
没有c

2. IdentityHashMap

• 键只以地址去重，而不是比较hashcode和equals
• 注意：键是常量池中的字符串

public void test4() {

    IdentityHashMap<String,String> map = new IdentityHashMap<>();

    map.put("a","a");
    map.put("a","a");

    int a = map.size();

    map.put(new String("c"),"c");
    map.put(new String("d"),"d");

    int b = map.size();
}

a = 1;
b = 3;

3. EnumMap

• 键必须为枚举的值
• 构造器：public EnumMap(指定枚举class对象)

public void test5() {

    EnumMap<Season,String> map = new EnumMap<Season, String>(Season.class);

    map.put(Season.SPRING,"春困");
    map.put(Season.SUMMER,"夏无力");
    map.put(Season.AUTUMN,"秋乏");
    map.put(Season.WINTER,"冬眠");

    int a = map.size();
}

a = 4

enum Season {
    SPRING,SUMMER,AUTUMN,WINTER
}