1.背景
hdfs短路读利用UNIX域套接字，可以让客户端和DataNode通信（客户端与dn是同一台机器）。需要在datanode和客户端的配置中都配置一个套接字路径，并且开启短路读特性。官方链接
hdfs在短路读时， 2.7版本的代码中DomainSocketWatcher（linux机器进程的socket通信的监听）类中有个bug需要修复。分析如下：
//DomainSocketWatcher#add相关代码
public void add(DomainSocket sock, Handler handler) {
lock.lock();
try {
Entry entry = new Entry(sock, handler);
toAdd.add(entry);
while (true) {
try {
processedCond.await();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
if (!toAdd.contains(entry)) {
break;
}
}
} finally {
lock.unlock();
}
}
//DomainSocketWatcher#remove相关代码
public void remove(DomainSocket sock) {
lock.lock();
try {
toRemove.put(sock.fd, sock);
while (true) {
try {
processedCond.await();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
if (!toRemove.containsKey(sock.fd)) {
break;
}
}
} finally {
lock.unlock();
}
}
final Thread watcherThread = new Thread(new Runnable() {
@Override
public void run() {
final TreeMap<Integer, Entry> entries = new TreeMap<Integer, Entry>();
try {
while (true) {
lock.lock();
try {
// Handle pending removals
while (true) {
Map.Entry<Integer, DomainSocket> entry = toRemove.firstEntry();
if (entry == null) break;
sendCallbackAndRemove("handlePendingRemovals", entries, fdSet, entry.getValue().fd);
}
processedCond.signalAll();
}
...
}
上述add / remove / watcherThread 是典型的线程通信代码。在 add / remove 中，注意以下代码：
while (true) {
try {
processedCond.await();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
...
线程如果捕获到中断，则再次通过Thread.currentThread().interrupt();中断当前线程，由于while(true)会进入下一次循环，执行processedCond.await();等待，进入await代码：
public final void await() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
...
如果线程已经被中断了，则抛出中断异常，会再次被InterruptedException捕获。从而陷入死循环。
2.测试
首先定义一个线程通信类Service，里面定义三个方法，分别用于测试await、awaitUninterruptibly、signalAll方法。具体如下：
public class Service {
private ReentrantLock lock = new ReentrantLock();
private Condition condition = lock.newCondition();
public void testAwaitMethod(){
try {
while (true) {
try {
lock.lock();
System.out.println("await begin");
condition.await();
System.out.println("await end");
} catch (InterruptedException e) {
e.printStackTrace();
System.out.println("catch await");
try {
Thread.sleep(2000);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
Thread.currentThread().interrupt();
}
}
}finally {
lock.unlock();
}
}
public void testAwaitUninterMethod(){
try {
while (true) {
try {
lock.lock();
System.out.println("awaitUninterruptibly begin");
condition.awaitUninterruptibly();
System.out.println("awaitUninterruptibly end");
} catch (Exception e) {
e.printStackTrace();
System.out.println("catch awaitUninterruptibly");
try {
Thread.sleep(2000);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
Thread.currentThread().interrupt();
}
}
} finally {
lock.unlock();
}
}
public void testSignalAll(){
try {
System.out.println("signalAll init");
lock.lock();
System.out.println("signalAll begin");
condition.signalAll();
System.out.println("signalAll end");
} catch (Exception e){
e.printStackTrace();
System.out.println("catch signalAll");
}finally {
lock.unlock();
}
}
}
定义两个线程类分别调用await()方法和awaitUninterruptibly(),如下：
public class MyThread0 extends Thread {
private Service service;
public MyThread0(Service service) {
this.service = service;
}
@Override
public void run() {
service.testAwaitMethod();
}
}
public class MyThread1 extends Thread {
private Service service;
public MyThread1(Service service) {
this.service = service;
}
@Override
public void run() {
service.testAwaitUninterMethod();
}
}
测试1：两个线程均await状态
public class Run {
public static void main(String[] args) {
try {
Service service = new Service();
MyThread0 myThread0 = new MyThread0(service);
myThread0.start();
MyThread1 myThread1 = new MyThread1(service);
myThread1.start();
} catch (Exception e) {
e.printStackTrace();
}
}
}
结果：
await begin
awaitUninterruptibly begin
测试2：signalAll后两线程再次await状态
public class Run {
public static void main(String[] args) {
try {
Service service = new Service();
MyThread0 myThread0 = new MyThread0(service);
myThread0.start();
MyThread1 myThread1 = new MyThread1(service);
myThread1.start();
Thread.sleep(100);
service.testSignalAll();
} catch (Exception e) {
e.printStackTrace();
}
}
}
结果：
await begin
awaitUninterruptibly begin
signalAll init
signalAll begin
signalAll end
await end
await begin
awaitUninterruptibly end
awaitUninterruptibly begin
测试3：显示调用interrupt()
显示调用interrupt()时，thread0进入catch，然后进入死循环；thread1直接忽略，不会进入catch。thread0进入死循环后，testSignalAll无法调用signalAll方法。
public class Run {
public static void main(String[] args) {
try {
Service service = new Service();
MyThread0 myThread0 = new MyThread0(service);
myThread0.start();
myThread0.interrupt();
MyThread1 myThread1 = new MyThread1(service);
myThread1.start();
myThread1.interrupt();
Thread.sleep(100);
service.testSignalAll();
} catch (Exception e) {
e.printStackTrace();
}
}
}
结果：
await begin
java.lang.InterruptedException
catch await
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2034)
at thread.lock.awaituninterruptibly.Service.testAwaitMethod(Service.java:22)
at thread.lock.awaituninterruptibly.MyThread0.run(MyThread0.java:15)
signalAll init
await begin
catch await
java.lang.InterruptedException
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2034)
at thread.lock.awaituninterruptibly.Service.testAwaitMethod(Service.java:22)
at thread.lock.awaituninterruptibly.MyThread0.run(MyThread0.java:15)
java.lang.InterruptedException
await begin
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2034)
catch await
at thread.lock.awaituninterruptibly.Service.testAwaitMethod(Service.java:22)
at thread.lock.awaituninterruptibly.MyThread0.run(MyThread0.java:15)
await begin
java.lang.InterruptedException
catch await
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2034)
at thread.lock.awaituninterruptibly.Service.testAwaitMethod(Service.java:22)
at thread.lock.awaituninterruptibly.MyThread0.run(MyThread0.java:15)
小结
通过上述测试3表明，使用 condition.awaitUninterruptibly(); 代替 condition.await();会忽略中断。另外，在while中，出现await和Thread.currentThread().interrupt();是不合理的。社区代码HADOOP-14214中已换成awaitUninterruptibly，并去掉Thread.currentThread().interrupt();