[Java] 浅谈Java引用和Threadlocal的那些事

user zhangsan = new user("zhangsan", 24);

user zhangsan;
zhangsan = new user("zhangsan", 24);

public softreference(t referent);
public softreference(t referent, referencequeue<? super t> q);

softzhangsan = new softreference(new user("zhangsan", 24));

public static void main(string[] args) {
 referencequeue<user> referencequeue = new referencequeue();
 softreference softreference = new softreference(new user("zhangsan",24), referencequeue);
 //手动触发gc
 system.gc();
 thread.sleep(1000);
 system.out.println("手动触发gc:" + softreference.get());
 system.out.println("手动触发的队列:" + referencequeue.poll());
 //通过堆内存不足触发gc
 makeheapnotenough();
 system.out.println("通过堆内存不足触发gc:" + softreference.get());
 system.out.println("通过堆内存不足触发gc:" + referencequeue.poll());
 }
 
 private static void makeheapnotenough() {
 softreference softreference = new softreference(new byte[1024*1024*5]);
 byte[] bytes = new byte[1024*1024*5];
 }
 输出:
 手动触发gc:user{name='zhangsan', age=24}
 手动触发的队列:null
 通过堆内存不足触发gc:null
 通过堆内存不足触发gc:java.lang.ref.softreference@4b85612c

soft references are most often used to implement memory-sensitive caches

public static void main(string[] args) {
 weakreference weakreference = new weakreference(new user("zhangsan",24));
 system.gc();
 system.out.println("手动触发gc:" + weakreference.get());
 }
输出结果:
手动触发gc:null

weak references are most often used to implement canonicalizing mappings.

public static void main(string[] args) throws exception {
 weakhashmap<user, string> weakhashmap = new weakhashmap();
 //强引用
 user zhangsan = new user("zhangsan", 24);
 weakhashmap.put(zhangsan, "zhangsan");
 system.out.println("有强引用的时候:map大小" + weakhashmap.size());
 //去掉强引用
 zhangsan = null;
 system.gc();
 thread.sleep(1000);
 system.out.println("无强引用的时候:map大小"+weakhashmap.size());
 }
输出结果为:
有强引用的时候:map大小1
无强引用的时候:map大小0

public static void main(string[] args) throws exception {
 referencequeue referencequeue = new referencequeue();
 phantomreference phantomreference = new phantomreference(new user("zhangsan", 24), referencequeue);
 system.out.println("什么也不做，获取:" + phantomreference.get());
 }
输出结果:
什么也不做，获取:null

phantom references are most often used for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the java finalization mechanism.

public static void main(string[] args) throws exception {
 cleaner.create(new user("zhangsan", 24), () -> {system.out.println("我被回收了，当前线程:{}"+ thread.currentthread().getname());});
 system.gc();
 thread.sleep(1000);
 }
输出:
我被回收了，当前线程:reference handler

static class entry extends weakreference<threadlocal<?>> {
/** the value associated with this threadlocal. */
object value;
 
entry(threadlocal<?> k, object v) {
super(k);
value = v;
}
}