java安全編碼指南之:Thread API調用規則

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"簡介"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"java中多線程的開發中少不了使用Thread，我們在使用Thread中提供的API過程中，應該注意些什麼規則呢？"}]},{"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":"一起來看一看吧。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"start一個Thread"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Thread中有兩個方法，一個是start方法，一個是run方法，兩個都可以調用，那麼兩個有什麼區別呢？"}]},{"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":"先看一下start方法："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"public synchronized void start() {\n\n if (threadStatus != 0)\n throw new IllegalThreadStateException();\n group.add(this);\n boolean started = false;\n try {\n start0();\n started = true;\n } finally {\n try {\n if (!started) {\n group.threadStartFailed(this);\n }\n } catch (Throwable ignore) {\n }\n }\n }\n\n private native void start0();\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":"start()是一個synchronized的方法，通過它會去調用native的start0方法，而最終將會調用Thread的run()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":"我們知道，創建一個Thread有兩種方式，一種是傳入一個Runnable，一個是繼承Thread，並重寫run()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":"如果我們直接調用Thread的run()方法會發生什麼事情呢？"}]},{"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":"先看一下run方法的定義："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void run() {\n if (target != null) {\n target.run();\n }\n }\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":"默認情況下， 這個target就是一個Runnable對象，如果Thread是通過Runnable來構建的話，調用Thread.run()會在當前線程中運行run方法中的內容。"}]},{"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":"如果Thread是以其形式構建，並且沒有重新run()方法，那麼直接調用Thread.run()將什麼都不會做。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void wrongStart(){\n Runnable runnable= ()-> System.out.println(\"in thread running!\");\n Thread thread= new Thread(runnable);\n thread.run();\n }\n\n public void correctStart(){\n Runnable runnable= ()-> System.out.println(\"in thread running!\");\n Thread thread= new Thread(runnable);\n thread.start();\n }\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":"所以，上面兩種調用方式，只有第二種是正確的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"不要使用ThreadGroup"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Thread中有個字段類型是java.lang.ThreadGroup，這個主要是用來給Thread進行分組，我們看下Thread的這個構造函數："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public Thread(ThreadGroup group, Runnable target) {\n this(group, target, \"Thread-\" + nextThreadNum(), 0);\n }\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":"上面的構造函數可以在傳入runnable的同時傳遞一個ThreadGroup對Thread進行分組。"}]},{"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":"如果沒有指定ThreadGroup，那麼將會爲其分配一個默認的default group。"}]},{"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":"ThreadGroup是做什麼的呢？ThreadGroup是java 1.0引入的方法，主要是一次性的對一組thread進行操作。我們可以調用ThreadGroup.interrupt()來一次性的對整個Group的Thread進行interrupts操作。"}]},{"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":"雖然ThreadGroup提供了很多有用的方法，但是其中很多方法都被廢棄了，比如：allowThreadSuspension(), resume(), stop(), 和 suspend()，並且ThreadGroup中還有很多方法是非線程安全的："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ThreadGroup.activeCount()"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個方法主要是用來統計一個ThreadGroup中活動的線程個數，這個方法會統計還未啓動的線程，同時也會受系統線程的影響，所以是不準確的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ThreadGroup.enumerate()"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個方法是將ThreadGroup和子group的線程拷貝到一個數組中，但是如果數組太小了，多餘的線程是會被自動忽略的。"}]},{"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":"ThreadGroup本身有一個 stop() 方法用來停止所有的線程，但是stop是不安全的，已經被廢棄了。"}]},{"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":"那麼我們該怎麼去安全的停止很多個線程呢？"}]},{"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":"使用executor.shutdown()就可以了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"不要使用stop()方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"剛剛講了ThreadGroup中不要調用stop()方法，因爲stop是不安全的。"}]},{"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":"調用stop方法會立馬釋放線程持有的所有的鎖，並且會拋出ThreadDeath異常。"}]},{"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":"因爲會釋放所有的鎖，所以可能會造成受這些鎖保護的對象的狀態發生不一致的情況。"}]},{"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":"替代的方法有兩種，一種是使用volatile flag變量，來控制線程的循環執行："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" private volatile boolean done = false;\n\n public void shutDown(){\n this.done= true;\n }\n\n public void stopWithFlag(){\n\n Runnable runnable= ()->{\n while(!done){\n System.out.println(\"in Runnable\");\n }\n };\n\n Thread thread= new Thread(runnable);\n thread.start();\n shutDown();\n }\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":"另外一種方法就是調用interrupt(), 這裏我們要注意interrupt()的使用要點："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"如果當前線程實例在調用Object類的wait（），wait（long）或wait（long，int）方法或join（），join（long），join（long，int）方法，或者在該實例中調用了Thread.sleep（long）或Thread.sleep（long，int）方法，並且正在阻塞狀態中時，則其中斷狀態將被清除，並將收到InterruptedException。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"如果此線程在InterruptibleChannel上的I/O操作中處於被阻塞狀態，則該channel將被關閉，該線程的中斷狀態將被設置爲true，並且該線程將收到java.nio.channels.ClosedByInterruptException異常。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"如果此線程在java.nio.channels.Selector中處於被被阻塞狀態，則將設置該線程的中斷狀態爲true，並且它將立即從select操作中返回。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"如果上面的情況都不成立，則設置中斷狀態爲true。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"先看下面的例子："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public static void main(String[] args) {\n Runnable runnable= ()->{\n while (!Thread.interrupted()) {\n System.out.println(\"in thread\");\n }\n };\n Thread thread= new Thread(runnable);\n thread.start();\n Thread.sleep(5000);\n thread.interrupt();\n }\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":"我們在while循環中調用了Thread.interrupted()方法用來判斷線程是否被設置了中斷位，然後在main方法中調用了thread.interrupt()來設置中斷，最終可以正確的停止Thread。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"注意，這裏運行的Thread並沒有被阻塞，所以並不滿足我們上面提到的第一個條件。"}]}]},{"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":"下面我們再看一個例子："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public static void main(String[] args) {\n Runnable runnable= ()->{\n while (!Thread.interrupted()) {\n System.out.println(\"in thread\");\n try {\n Thread.sleep(5000);\n } catch (InterruptedException e) {\n e.printStackTrace();\n }\n }\n };\n\n Thread thread= new Thread(runnable);\n thread.start();\n thread.interrupt();\n }\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":"這個例子和上面的例子不同之處就是在於，Thread中調用了sleep方法，導致Thread被阻塞了，最終滿足了第一個條件，從而不會設置終端位，只會拋出InterruptedException，所以這個例子中線程是不會被停止的，大家一定要注意。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"wait 和 await 需要放在循環中調用"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲什麼要放在循環中呢？因爲我們希望wait不是被錯誤的被喚醒，所以我們需要在wait被喚醒之後，重新檢測一遍條件。"}]},{"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":"錯誤的調用是放在if語句中："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"synchronized (object) {\n if () {\n object.wait();\n }\n // Proceed when condition holds\n}\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":"正確的方法是放在while循環中："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"synchronized (object) {\n while () {\n object.wait();\n }\n // Proceed when condition holds\n}\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":"本文的代碼："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://github.com/ddean2009/learn-java-base-9-to-20/tree/master/security","title":null},"content":[{"type":"text","text":"learn-java-base-9-to-20/tree/master/security"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"本文已收錄於 "},{"type":"link","attrs":{"href":"http://www.flydean.com/java-security-code-line-thread/","title":null},"content":[{"type":"text","text":"http://www.flydean.com/java-security-code-line-thread/"}],"marks":[{"type":"italic"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"最通俗的解讀，最深刻的乾貨，最簡潔的教程，衆多你不知道的小技巧等你來發現！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"歡迎關注我的公衆號:「程序那些事」,懂技術，更懂你！"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}