生產中遇到的Java多執行緒問題
阿新 • • 發佈：2018-12-16
1. 問題場景：前幾天一位同事分享了一段程式碼，這段程式碼在線上偶爾會報空指標異常，雖然是一個簡單的NullPointerException，卻是一個很不容易發現的多執行緒問題導致的，我們一起先來看下程式碼（這是我復原的測試程式碼，並非和生產一致，但整體邏輯是一樣的）：
public class TestSingleton { private static volatile TestSingleton testSingleton = null; static Map<String, String> idCard; private TestSingleton() {} public static TestSingleton init() { if (testSingleton == null) { synchronized (TestSingleton.class) { if (testSingleton == null) { testSingleton = new TestSingleton(); idCard = new HashMap<>(); idCard.put("aa", "bb"); } } } return testSingleton; } public String getIdCard() { return idCard.get("aa"); } public static void main(String[] args) { TestSingleton testSingleton = TestSingleton.init().getIdCard(); } }
整體來看這位仁兄是實現了一個單例類，看樣子沒什麼問題，只是多了一個Map來例項化並設定一些kv值，從邏輯上來沒看出有什麼毛病，我一開始看到這個程式碼確實沒看出什麼問題來。
如果先不看後續的描述，你能看出問題來嗎？
靜靜地觀察一分鐘...
2. 問題還原分析：通過觀察看出是什麼問題了嗎？如果你立馬定位了原因，那麼說明你的多執行緒還是挺紮實的，接下來我們一塊來分析一下問題原因。首先我啟動main方法執行，沒有發現任何問題，多試幾次也沒問題，但這並不能說明就真正沒問題（要不然生產上的問題就沒法解釋了），因為我現在這樣執行main方法是單執行緒，只能說明這個程式碼在單執行緒場景下是沒有任何問題的。現在我將main方法改成多線執行：
public static void main(String[] args) { int n = 10; CountDownLatch countDownLatch = new CountDownLatch(n); ExecutorService executorService = Executors.newFixedThreadPool(n); for (int i = 0; i < n; i++) { executorService.execute(() -> { TestSingleton.init().getIdCard(); countDownLatch.countDown(); }); } try { //等待所有執行緒執行完畢 countDownLatch.await(); } catch (InterruptedException e) { e.printStackTrace(); } //統計一下執行情況，看是否都執行完成 ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) executorService; System.out.println("總任務數：" + threadPoolExecutor.getTaskCount()); System.out.println("正在執行任務數字：" + threadPoolExecutor.getActiveCount()); System.out.println("完成任務數：" + threadPoolExecutor.getCompletedTaskCount()); }
接下來我使用執行緒池，啟動10個執行緒試試看能否還原出報NullPointException，多執行幾次發現確實報出了空指標，
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/bin/java "-javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=52108:/Applications/IntelliJ IDEA.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/tools.jar:/Users/workspace/ideaSpace/jwt-helper/target/classes:/Users/workspace/mavenRepository/com/auth0/java-jwt/3.3.0/java-jwt-3.3.0.jar:/Users/workspace/mavenRepository/com/fasterxml/jackson/core/jackson-databind/2.9.2/jackson-databind-2.9.2.jar:/Users/workspace/mavenRepository/com/fasterxml/jackson/core/jackson-annotations/2.9.0/jackson-annotations-2.9.0.jar:/Users/workspace/mavenRepository/com/fasterxml/jackson/core/jackson-core/2.9.2/jackson-core-2.9.2.jar:/Users/workspace/mavenRepository/io/jsonwebtoken/jjwt/0.9.0/jjwt-0.9.0.jar:/Users/workspace/mavenRepository/com/alibaba/fastjson/1.2.47/fastjson-1.2.47.jar:/Users/workspace/mavenRepository/org/apache/commons/commons-lang3/3.7/commons-lang3-3.7.jar:/Users/workspace/mavenRepository/org/slf4j/slf4j-log4j12/1.6.4/slf4j-log4j12-1.6.4.jar:/Users/workspace/mavenRepository/log4j/log4j/1.2.16/log4j-1.2.16.jar:/Users/workspace/mavenRepository/org/slf4j/slf4j-api/1.7.20/slf4j-api-1.7.20.jar:/Users/workspace/mavenRepository/commons-codec/commons-codec/1.11/commons-codec-1.11.jar com.chtwm.component.secret.TestSingleton
Exception in thread "pool-1-thread-2" Exception in thread "pool-1-thread-3" java.lang.NullPointerException
at com.chtwm.component.secret.TestSingleton.getIdCard(TestSingleton.java:43)
at com.chtwm.component.secret.TestSingleton.lambda$main$0(TestSingleton.java:52)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
java.lang.NullPointerException
at com.chtwm.component.secret.TestSingleton.getIdCard(TestSingleton.java:43)
at com.chtwm.component.secret.TestSingleton.lambda$main$0(TestSingleton.java:52)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Process finished with exit code 130 (interrupted by signal 2: SIGINT)報錯位置是getIdCard()方法獲取idCard中資訊的時候，
public String getIdCard() {
return idCard.get("aa");
}說明是idCard這個map是Null（一開始我的猜想也是這個map物件可能存在問題，因為除了Map相關的操作程式碼，其他程式碼是標準的單例實現模式，我之前也經常使用這樣的模式），那麼什麼情況會導致Map物件為空了呢？
我們再回頭捋一下程式碼，在多執行緒情況下，有這樣的場景會造成idCard物件為Null：
假如有一個執行緒A進入到了同步鎖，且剛執行完
testSingleton = new TestSingleton();操作，還未進行下一步例項化Map物件。此時，另一個執行緒B同時在判斷testSingleton是否為Null，
if (testSingleton == null)因為testSingleton物件是被volatile修飾的，volatile修復的變數具有多執行緒可見性、防指令重排序的特性，所以此時testSingleton是非空的，那麼將直接返回testSingleton物件呼叫getIdCard()，但此時Map物件idCard又還沒有例項化，是一個null值，因此在取map物件中key值的時候導致了報出NullPointException。
3. 問題驗證：現在你應該大概明白是什麼原因了吧，為了再次驗證一下我們分析的結果，我把執行緒數設定成2個，然後在例項化idCard物件前休眠5秒鐘，然後再加一些日誌列印，更能清晰的說明問題，修改後的程式碼：
public static TestSingleton init() {
System.out.println("當前執行緒名稱>>" + Thread.currentThread().getName());
if (testSingleton == null) {
synchronized (TestSingleton.class) {
if (testSingleton == null) {
testSingleton = new TestSingleton();
System.out.println(Thread.currentThread().getName() + "[持有鎖並例項化完成testSingleton物件==開始休眠...]");
//休眠一會
try {
TimeUnit.SECONDS.sleep(5);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "[休眠完成]");
//建立map
idCard = new HashMap<>();
idCard.put("aa", "bb");
}
}
}
System.out.println(Thread.currentThread().getName() + "[返回testSingleton物件，打印出物件:"+testSingleton+"]");
return testSingleton;
}
public String getIdCard() {
System.out.println(Thread.currentThread().getName() + "[從idCard物件獲取key值，列印idCard物件:"+idCard+"]");
return idCard.get("aa");
}設定2個執行緒併發，再次執行，這樣基本每次都會復現問題，以下是錯誤日誌資訊（日誌列印的順序可能有點錯亂，但不影響關鍵問題點的分析）：
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/bin/java "-javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=52197:/Applications/IntelliJ IDEA.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/tools.jar:/Users/workspace/ideaSpace/jwt-helper/target/classes:/Users/workspace/mavenRepository/com/auth0/java-jwt/3.3.0/java-jwt-3.3.0.jar:/Users/workspace/mavenRepository/com/fasterxml/jackson/core/jackson-databind/2.9.2/jackson-databind-2.9.2.jar:/Users/workspace/mavenRepository/com/fasterxml/jackson/core/jackson-annotations/2.9.0/jackson-annotations-2.9.0.jar:/Users/workspace/mavenRepository/com/fasterxml/jackson/core/jackson-core/2.9.2/jackson-core-2.9.2.jar:/Users/workspace/mavenRepository/io/jsonwebtoken/jjwt/0.9.0/jjwt-0.9.0.jar:/Users/workspace/mavenRepository/com/alibaba/fastjson/1.2.47/fastjson-1.2.47.jar:/Users/workspace/mavenRepository/org/apache/commons/commons-lang3/3.7/commons-lang3-3.7.jar:/Users/workspace/mavenRepository/org/slf4j/slf4j-log4j12/1.6.4/slf4j-log4j12-1.6.4.jar:/Users/workspace/mavenRepository/log4j/log4j/1.2.16/log4j-1.2.16.jar:/Users/workspace/mavenRepository/org/slf4j/slf4j-api/1.7.20/slf4j-api-1.7.20.jar:/Users/workspace/mavenRepository/commons-codec/commons-codec/1.11/commons-codec-1.11.jar com.chtwm.component.secret.TestSingleton
當前執行緒名稱>>pool-1-thread-1
當前執行緒名稱>>pool-1-thread-2
pool-1-thread-1[持有鎖並例項化完成testSingleton物件==開始休眠...]
pool-1-thread-2[返回testSingleton物件，打印出物件:[email protected]]
Exception in thread "pool-1-thread-2" java.lang.NullPointerException
pool-1-thread-2[從idCard物件獲取key值，列印idCard物件:null]
at com.chtwm.component.secret.TestSingleton.getIdCard(TestSingleton.java:54)
at com.chtwm.component.secret.TestSingleton.lambda$main$0(TestSingleton.java:63)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
pool-1-thread-1[休眠完成]
pool-1-thread-1[返回testSingleton物件，打印出物件:[email protected]]
pool-1-thread-1[從idCard物件獲取key值，列印idCard物件:{aa=bb}]從錯誤資訊中就可以看出，執行緒1持有了同步鎖並例項化了testSingleton物件，然後開始休眠，idCard並沒有例項化。此時執行緒2直接返回了testSingleton物件，呼叫了getIdCard()方法，但idCard物件還是null，所以就報出了空指標異常，驗證完畢。
4. 問題解決：找到了問題點，那麼解決起來就比較容易了，想必應該有多種方式可以解決這個問題。以下是相對簡單的解決方式：
- 將synchronized關鍵字放到init()方法上，這樣同時只能有一個執行緒進入到該方法，從而就能避免該問題；
public static synchronized TestSingleton init() {
if (testSingleton == null) {
testSingleton = new TestSingleton();
//建立map
idCard = new HashMap<>();
idCard.put("aa", "bb");
}
return testSingleton;
}- 將idCard的例項化並賦值的操作放入到構造方法裡，在例項化單例物件的同時也例項化了Map物件；
private TestSingleton() {
idCard = new HashMap<>();
idCard.put("aa", "bb");
}