[A] map: WeakHashMap

Author: c-rainstorm

java/source-code/jdk/collection/map-weakhashmap.md

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+# `WeakHashMap<K,V>`
+
+## 设计
+
+链地址法，不转红黑树。
+
+使用 `WeakReference` 做 `Entry` 利用 GC 来回收条目
+
+## 扩容策略
+
+```java
+/**
+* The default initial capacity -- MUST be a power of two.
+*/
+private static final int DEFAULT_INITIAL_CAPACITY = 16;
+
+/**
+* The load factor used when none specified in constructor.
+*/
+private static final float DEFAULT_LOAD_FACTOR = 0.75f;
+```
+
+达到阈值直接扩容为原来两倍
+
+```java
+if (++size >= threshold)
+    resize(tab.length * 2);
+```
+
+## 开源组件拓展
+
+### Tomcat
+
+```java
+
+public final class ConcurrentCache<K,V> {
+
+    private final int size;
+
+    private final Map<K,V> eden;
+
+    private final Map<K,V> longterm;
+
+    public ConcurrentCache(int size) {
+        this.size = size;
+        this.eden = new ConcurrentHashMap<>(size);
+        this.longterm = new WeakHashMap<>(size);
+    }
+
+    public V get(K k) {
+        V v = this.eden.get(k);
+        if (v == null) {
+            synchronized (longterm) {
+                v = this.longterm.get(k);
+            }
+            if (v != null) {
+                this.eden.put(k, v);
+            }
+        }
+        return v;
+    }
+
+    public void put(K k, V v) {
+        if (this.eden.size() >= size) {
+            synchronized (longterm) {
+                this.longterm.putAll(this.eden);
+            }
+            this.eden.clear();
+        }
+        this.eden.put(k, v);
+    }
+}
+```
+
+## 参考
+
+- [Java Reference - TODO](../lang/ref/reference.md)
+- [WeakHashMap 的实际使用场景有哪些？ - 知乎](https://www.zhihu.com/question/46648593)
+- [ConcurrentCache - Tomcat](https://github.com/apache/tomcat/blob/trunk/java/org/apache/tomcat/util/collections/ConcurrentCache.java)

java/source-code/jdk/lang/ref/reference.md

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+# Reference
+
+// todo 问题太多，暂时搁置
+
+## 参考
+
+- [java中的引用 - KL's blog](https://qsli.github.io/2018/07/08/reference/)
+- [强软弱虚引用，只有体会过了，才能记住 - CoderBear](https://juejin.im/post/5e65b8096fb9a07cbb6e4a43)
+- [回过头来看对象的四种状态强软弱虚引用的理解 - 小勇DW3](https://cloud.tencent.com/developer/article/1198496)
+- [进击的Java新人](https://zhuanlan.zhihu.com/p/24393775?refer=hinus)
+- [Reference Handler线程的作用](https://zhuanlan.zhihu.com/p/40559078)

res/java-reference-instance-state-transfer.pu

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+@startuml java-reference-instance-state-transfer
+
+' Active
+' Pending
+' Enqueued
+' Inactive
+
+[*] --> Active
+Active : 新创建的 Reference 实例\nnew SoftReference()
+
+note bottom of Pending
+
+SoftReference - GC 时触发状态变更
+WeakReference - 非 GC，但是被GC扫描线程扫到了
+
+end note
+
+Active --> Pending : by GC
+
+Pending : 表示引用可以被回收了，\n通知用户层对引用做最后的处理
+
+Active --> Inactive : by GC
+
+Pending -right-> Enqueued : by ReferenceHandler thread
+
+Enqueued -right-> Inactive : 从 ReferenceQueue 移除时\n 一般由用户线程处理\n比如 WeakHashMap#expungeStaleEntries
+
+Inactive : 这个状态 GC 会释放对象
+
+Inactive --> [*]
+
+
+caption Java引用实例状态转换
+
+right footer https://github.com/c-rainstorm/blog 转载请注明出处
+
+@enduml