java 引用

[转]Java 的强引用、弱引用、软引用、虚引用
https://www.cnblogs.com/gudi/p/6403953.html

java集合（四）Map集合之WeakHashMap详解
https://www.cnblogs.com/yuexiaoyun/articles/12189128.html

Java中关于WeakReference和WeakHashMap的理解
https://www.cnblogs.com/dennyzhangdd/p/7496260.html

Java WeakReference的理解与使用
https://blog.csdn.net/hikvision_java_gyh/article/details/53260585