Java四種引用類型：強引用、軟引用、弱引用、虛引用

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"Java爲什麼要設計這四種引用"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Java的內存分配和內存回收，都不需要程序員負責，都是由偉大的JVM去負責，一個對象是否可以被回收，主要看是否有引用指向此對象，說的專業點，叫可達性分析。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Java設計這四種引用的主要目的有兩個："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"可以讓程序員通過代碼的方式來決定某個對象的生命週期；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"有利用垃圾回收。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"強引用"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"強引用是最普遍的一種引用，我們寫的代碼，99.9999%都是強引用："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"Object o = new Object();"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這種就是強引用了，是不是在代碼中隨處可見，最親切。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"只要某個對象有強引用與之關聯，這個對象永遠不會被回收，即使內存不足，JVM寧願拋出OOM，也不會去回收。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那麼什麼時候纔可以被回收呢？當強引用和對象之間的關聯被中斷了，就可以被回收了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們可以手動把關聯給中斷了，方法也特別簡單："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"o = null;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們可以手動調用GC，看看如果強引用和對象之間的關聯被中斷了，資源會不會被回收，爲了更方便、更清楚的觀察到回收的情況，我們需要新寫一個類，然後重寫finalize方法，下面我們來進行這個實驗："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class Student { @Override\n protected void finalize() throws Throwable {\n System.out.println(\"Student 被回收了\");\n }}public static void main(String[] args) {\n Student student = new Student();\n student = null;\n System.gc();}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"運行結果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"Student 被回收了"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以很清楚的看到資源被回收了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"},{"type":"strong"}],"text":"當然，在實際開發中，千萬不要重寫finalize方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在實際的開發中，看到有一些對象被手動賦值爲NULL，很大可能就是爲了“特意提醒”JVM這塊資源可以進行垃圾回收了。點擊這裏獲取一份 JVM 實戰教程。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"軟引用"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面先來看看如何創建一個軟引用："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" SoftReferencestudentSoftReference=new SoftReference(new Student());"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"軟引用就是把對象用SoftReference包裹一下，當我們需要從軟引用對象獲得包裹的對象，只要get一下就可以了："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"SoftReferencestudentSoftReference=new SoftReference(new Student());\nStudent student = studentSoftReference.get();\nSystem.out.println(student);"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"軟引用有什麼特點呢："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當內存不足，會觸發JVM的GC，如果GC後，內存還是不足，就會把軟引用的包裹的對象給幹掉，也就是隻有在內存不足，JVM纔會回收該對象。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"還是一樣的，必須做實驗，才能加深印象："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"SoftReference softReference = new SoftReference(new byte[1024*1024*10]);\nSystem.out.println(softReference.get());\nSystem.gc();System.out.println(softReference.get()); \nbyte[] bytes = new byte[1024 * 1024 * 10];\nSystem.out.println(softReference.get());"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我定義了一個軟引用對象，裏面包裹了byte[]，byte[]佔用了10M，然後又創建了10Mbyte[]。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"運行程序，需要帶上一個參數："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"-Xmx20M"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"代表最大堆內存是20M。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"運行結果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"[B@11d7fff\n[B@11d7fff\nnull"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以很清楚的看到手動完成GC後，軟引用對象包裹的byte[]還活的好好的，但是當我們創建了一個10M的byte[]後，最大堆內存不夠了，所以把軟引用對象包裹的byte[]給幹掉了，如果不幹掉，就會拋出OOM。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"軟引用到底有什麼用呢？比較適合用作緩存，當內存足夠，可以正常的拿到緩存，當內存不夠，就會先幹掉緩存，不至於馬上拋出OOM。說到緩存，大家可以關注微信公衆號Java技術棧獲取更多幹貨。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"弱引用"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"弱引用的使用和軟引用類似，只是關鍵字變成了WeakReference："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"WeakReference weakReference = new WeakReference(new byte[1024\\*1024\\*10]);\nSystem.out.println(weakReference.get());"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"弱引用的特點是不管內存是否足夠，只要發生GC，都會被回收："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"WeakReference weakReference = new WeakReference(new byte[1]);\nSystem.out.println(weakReference.get());\nSystem.gc();System.out.println(weakReference.get());"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"運行結果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"[B@11d7fffnull"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以很清楚的看到明明內存還很充足，但是觸發了GC，資源還是被回收了。 "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"弱引用在很多地方都有用到，比如ThreadLocal、WeakHashMap。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"虛引用"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"虛引用又被稱爲幻影引用，我們來看看它的使用："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"ReferenceQueue queue = new ReferenceQueue();\nPhantomReference reference = new PhantomReference(new byte[1], queue);\nSystem.out.println(reference.get());"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"虛引用的使用和上面說的軟引用、弱引用的區別還是挺大的，我們先不管ReferenceQueue 是個什麼鬼，直接來運行："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"null"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"竟然打印出了null，我們來看看get方法的源碼："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public T get() { \n　　return null;\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這是幾個意思，竟然直接返回了null。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這就是虛引用特點之一了：無法通過虛引用來獲取對一個對象的真實引用。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那虛引用存在的意義是什麼呢？這就要回到我們上面的代碼了，我們把代碼複製下，以免大家再次往上翻："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"ReferenceQueue queue = new ReferenceQueue();\nPhantomReference reference = new PhantomReference(new byte[1], queue);\nSystem.out.println(reference.get());"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"創建虛引用對象，我們除了把包裹的對象傳了進去，還傳了一個ReferenceQueue，從名字就可以看出它是一個隊列。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"虛引用的特點之二就是 虛引用必須與ReferenceQueue一起使用，當GC準備回收一個對象，如果發現它還有虛引用，就會在回收之前，把這個虛引用加入到與之關聯的ReferenceQueue中。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們來用代碼實踐下吧："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"ReferenceQueue queue = new ReferenceQueue();\nList bytes = new ArrayList<>();\nPhantomReference reference = new PhantomReference(new Student(),queue);\nnew Thread(() -> {\n for (int i = 0; i < 100;i++ ) {\n bytes.add(new byte[1024 * 1024]);\n }}).start();new Thread(() -> {\n while (true) {\n Reference poll = queue.poll(); if (poll != null) {\n System.out.println(\"虛引用被回收了：\" + poll);\n } }}).start();Scanner scanner = new Scanner(System.in);\nscanner.hasNext();}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"運行結果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"Student 被回收了\n虛引用被回收了：java.lang.ref.PhantomReference@1ade6f1"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們簡單的分析下代碼： "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第一個線程往集合裏面塞數據，隨着數據越來越多，肯定會發生GC。 "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第二個線程死循環，從queue裏面拿數據，如果拿出來的數據不是null，就打印出來。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"從運行結果可以看到：當發生GC，虛引用就會被回收，並且會把回收的通知放到ReferenceQueue中。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"虛引用有什麼用呢？在NIO中，就運用了虛引用管理堆外內存."}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}