强引用
常见的对象赋值引用，不为空，则不会被垃圾回收器回收直到内存溢出
demo
/**
* 强引用
*/
@Test
public void dd(){
Object o = new Object();
System.gc();
assert o != null;
o = null;
assert o == null;
}
GC可达性检测
软引用
次于强引用，但比强引用更弱，不会被垃圾回收器回收，但内存溢出时，会被回收
适合缓存，比如图片缓存，文件缓存，数据库连接缓存，缓存数据，图片等
/**
* 软引用
*/
@Test
public void dd1(){
SoftReference<Object> o = new SoftReference(new Object());
assert o != null;
}
弱引用
弱引用，比软引用更弱只能生存到下一次垃圾收集发生之前
ThreadLocalMap 源码结构
static class ThreadLocalMap {
/**
* The entries in this hash map extend WeakReference, using
* its main ref field as the key (which is always a
* ThreadLocal object). Note that null keys (i.e. entry.get()
* == null) mean that the key is no longer referenced, so the
* entry can be expunged from table. Such entries are referred to
* as "stale entries" in the code that follows.
*/
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
/**
* 弱引用
*/
@Test
public void dd2(){
WeakReference<Object> o = new WeakReference(new Object());
assert o != null;
}
虚引用
资源回收回调
/**
* 虚引用
*/
@Test
public void dd3(){
// 创建一个引用队列
ReferenceQueue<Object> queue = new ReferenceQueue<>();
}
