lucene代码分析7

[email protected]

有关 SimpleFSLock 进行 JVM 之间的同步

有时候，我们写 java 程序的时候，也需要不同的 JVM 之间进行同步，来保护一个整个系统中唯一的资源。
如果唯一的资源仅仅在一个进程中，则可以使用线程同步的机制，然而如果唯一的资源要被多个进程进行访问，则需要进程间同步的机制，无论是
Windows 和 Linux 在操作系统层面都有很多的进程间同步的机制。
但进程间的同步却不是 Java 的特长，Lucene 的 SimpleFSLock 给我们提供了一种方式。 如下：

Lock 的抽象类

public abstract class Lock { 
 public static long LOCK_POLL_INTERVAL = 1000; 
 public static final long LOCK_OBTAIN_WAIT_FOREVER = -1; 
 public abstract boolean obtain() throws IOException; 
 public boolean obtain(long lockWaitTimeout) throws LockObtainFailedException, IOException { 
 boolean locked = obtain(); 

 if (lockWaitTimeout < 0 && lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER) 
 throw new IllegalArgumentException("..."); 
 long maxSleepCount = lockWaitTimeout / LOCK_POLL_INTERVAL; 
 long sleepCount = 0; 
 while (!locked) { 
 if (lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER && sleepCount++ >= maxSleepCount) { 
 throw new LockObtainFailedException("Lock obtain timed out."); 
 } 
 try { 
 Thread.sleep(LOCK_POLL_INTERVAL); 
 } catch (InterruptedException ie) { 
 throw new ThreadInterruptedException(ie); 
 } 
 locked = obtain(); 
 } 
 return locked; 
 } 
 public abstract void release() throws IOException; 
 public abstract boolean isLocked() throws IOException; 
} 

LockFactory 的抽象类

public abstract class LockFactory { 
 public abstract Lock makeLock(String lockName); 
 abstract public void clearLock(String lockName) throws IOException; 
} 
SimpleFSLock 的实现类
class SimpleFSLock extends Lock { 
 File lockFile; 
 File lockDir; 
 public SimpleFSLock(File lockDir, String lockFileName) { 
 this.lockDir = lockDir; 
 lockFile = new File(lockDir, lockFileName); 
 } 
 @Override 
 public boolean obtain() throws IOException { 
 if (!lockDir.exists()) { 
 if (!lockDir.mkdirs()) 
 throw new IOException("Cannot create directory: " + lockDir.getAbsolutePath()); 
 } else if (!lockDir.isDirectory()) { 
 throw new IOException("Found regular file where directory expected: " + 
lockDir.getAbsolutePath()); 
 } 
 return lockFile.createNewFile(); 
 } 
 @Override 
 public void release() throws LockReleaseFailedException { 
 if (lockFile.exists() && !lockFile.delete()) 
 throw new LockReleaseFailedException("failed to delete " + lockFile); 
 } 
 @Override 
 public boolean isLocked() { 
 return lockFile.exists(); 
 } 
} 

SimpleFSLockFactory 的实现类

public class SimpleFSLockFactory extends FSLockFactory { 
public SimpleFSLockFactory(String lockDirName) throws IOException { 
 setLockDir(new File(lockDirName)); 
 } 
 @Override 
 public Lock makeLock(String lockName) { 
 if (lockPrefix != null) { 
 lockName = lockPrefix + "-" + lockName; 
 } 
 return new SimpleFSLock(lockDir, lockName); 
 } 
 @Override 
 public void clearLock(String lockName) throws IOException { 
 if (lockDir.exists()) { 
 if (lockPrefix != null) { 
 lockName = lockPrefix + "-" + lockName; 
 } 
 File lockFile = new File(lockDir, lockName); 
 if (lockFile.exists() && !lockFile.delete()) { 
 throw new IOException("Cannot delete " + lockFile); 
 } 
 } 
 } 
};