大家都知道，在多线程或多进程的环境中，对统一资源的访问需要特别小心，特别是在写资源时，如果不加锁，将会导致很多严重的后果，Lucene的索引也是如此，lucene对索引的读写分为IndexReader和IndexWriter，顾名思义，一个读，一个写，lucene可以对同一个索引文件建立多个IndexReader对象，但是只能有一个IndexWriter对象，这是怎么做到的呢？显而易见是需要加锁的，加锁可以保证一个索引文件只能建立一个IndexWriter对象。下面就细说Lucene索引文件锁机制：
如果我们对同一个索引文件建立多个不同的IndexWriter会怎么样呢？
IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer); IndexWriter indexWriter = new IndexWriter(dir, indexWriterConfig); IndexWriterConfig indexWriterConfig2 = new IndexWriterConfig(analyzer); IndexWriter indexWriter2 = new IndexWriter(dir,indexWriterConfig2);
运行后，控制台输出：
Exception in thread "main" org.apache.lucene.store.LockObtainFailedException: Lock obtain timed out: [email protected]:\Users\new\Desktop\Lucene\write.lock
at org.apache.lucene.store.Lock.obtain(Lock.java:89)
at org.apache.lucene.index.IndexWriter.<init>(IndexWriter.java:755)
at test.Index.index(Index.java:51)
at test.Index.main(Index.java:78)
显然是不可以对同一个索引文件开启多个IndexWriter。
上面是一个比较简略的类图，可以看到lucene采用了工厂方法，这样可以方便扩展其他实现，这里只以SimpleFsLock为例说明lucene的锁机制（其他的有兴趣可以看lucene源码）。
Lock类是锁的基类，一个抽象类，源码如下：
public abstract class Lock implements Closeable {
/** How long {@link #obtain(long)} waits, in milliseconds,
* in between attempts to acquire the lock. */
public static long LOCK_POLL_INTERVAL = 1000;
/** Pass this value to {@link #obtain(long)} to try
* forever to obtain the lock. */
public static final long LOCK_OBTAIN_WAIT_FOREVER = -1;
/** Attempts to obtain exclusive access and immediately return
* upon success or failure. Use {@link #close} to
* release the lock.
* @return true iff exclusive access is obtained
*/
public abstract boolean obtain() throws IOException;
/**
* If a lock obtain called, this failureReason may be set
* with the "root cause" Exception as to why the lock was
* not obtained.
*/
protected Throwable failureReason;
/** Attempts to obtain an exclusive lock within amount of
* time given. Polls once per {@link #LOCK_POLL_INTERVAL}
* (currently 1000) milliseconds until lockWaitTimeout is
* passed.
* @param lockWaitTimeout length of time to wait in
* milliseconds or {@link
* #LOCK_OBTAIN_WAIT_FOREVER} to retry forever
* @return true if lock was obtained
* @throws LockObtainFailedException if lock wait times out
* @throws IllegalArgumentException if lockWaitTimeout is
* out of bounds
* @throws IOException if obtain() throws IOException
*/
public final boolean obtain(long lockWaitTimeout) throws IOException {
failureReason = null;
boolean locked = obtain();
if (lockWaitTimeout < 0 && lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER)
throw new IllegalArgumentException("lockWaitTimeout should be LOCK_OBTAIN_WAIT_FOREVER or a non-negative number (got " + lockWaitTimeout + ")");
long maxSleepCount = lockWaitTimeout / LOCK_POLL_INTERVAL;
long sleepCount = 0;
while (!locked) {
if (lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER && sleepCount++ >= maxSleepCount) {
String reason = "Lock obtain timed out: " + this.toString();
if (failureReason != null) {
reason += ": " + failureReason;
}
throw new LockObtainFailedException(reason, failureReason);
}
try {
Thread.sleep(LOCK_POLL_INTERVAL);
} catch (InterruptedException ie) {
throw new ThreadInterruptedException(ie);
}
locked = obtain();
}
return locked;
}
/** Releases exclusive access. */
public abstract void close() throws IOException;
/** Returns true if the resource is currently locked. Note that one must
* still call {@link #obtain()} before using the resource. */
public abstract boolean isLocked() throws IOException;
/** Utility class for executing code with exclusive access. */
public abstract static class With {
private Lock lock;
private long lockWaitTimeout;
/** Constructs an executor that will grab the named lock. */
public With(Lock lock, long lockWaitTimeout) {
this.lock = lock;
this.lockWaitTimeout = lockWaitTimeout;
}
/** Code to execute with exclusive access. */
protected abstract Object doBody() throws IOException;
/** Calls {@link #doBody} while <i>lock</i> is obtained. Blocks if lock
* cannot be obtained immediately. Retries to obtain lock once per second
* until it is obtained, or until it has tried ten times. Lock is released when
* {@link #doBody} exits.
* @throws LockObtainFailedException if lock could not
* be obtained
* @throws IOException if {@link Lock#obtain} throws IOException
*/
public Object run() throws IOException {
boolean locked = false;
try {
locked = lock.obtain(lockWaitTimeout);
return doBody();
} finally {
if (locked) {
lock.close();
}
}
}
}
}
里面最重要的方法就是obtain()，这个方法用来维持锁，建立锁之后，维持时间为LOCK_POLL_INTERVAL，之后需要重新申请维持锁，这样做是为了支持多线程读写。当然也可以将lockWaitTimeout设置为-1，这样就是一直维持写锁。
抽象基类LockFactory，只定义了一个抽象方法makeLock，返回Lock对象的一个实例。
public abstract class LockFactory {
/**
* Return a new Lock instance identified by lockName.
* @param lockName name of the lock to be created.
*/
public abstract Lock makeLock(Directory dir, String lockName);
}
抽象类FSLockFactory继承Lock：
public abstract class FSLockFactory extends LockFactory {
/** Returns the default locking implementation for this platform.
* This method currently returns always {@link NativeFSLockFactory}.
*/
public static final FSLockFactory getDefault() {
return NativeFSLockFactory.INSTANCE;
}
@Override
public final Lock makeLock(Directory dir, String lockName) {
if (!(dir instanceof FSDirectory)) {
throw new UnsupportedOperationException(getClass().getSimpleName() + " can only be used with FSDirectory subclasses, got: " + dir);
}
return makeFSLock((FSDirectory) dir, lockName);
}
/** Implement this method to create a lock for a FSDirectory instance. */
protected abstract Lock makeFSLock(FSDirectory dir, String lockName);
}
可以看到
public static final FSLockFactory getDefault() {
return NativeFSLockFactory.INSTANCE;
}
这个方法默认返回NativeFSLockFactory，和SimpleFSLockFactory一样是一个具体实现，NativeFSLockFactory使用的是nio中FileChannel.tryLock方法，这里不展开讨论，有兴趣的读者可以去看jdk nio的源码（好像现在oracle不提供FileChannel实现类的源码了，需要去jvm里找）。
下面就是本篇文章的重头戏，SimpleFSLockFactory
public final class SimpleFSLockFactory extends FSLockFactory {
/**
* Singleton instance
*/
public static final SimpleFSLockFactory INSTANCE = new SimpleFSLockFactory();
private SimpleFSLockFactory() {}
@Override
protected Lock makeFSLock(FSDirectory dir, String lockName) {
return new SimpleFSLock(dir.getDirectory(), lockName);
}
static class SimpleFSLock extends Lock {
Path lockFile;
Path lockDir;
public SimpleFSLock(Path lockDir, String lockFileName) {
this.lockDir = lockDir;
lockFile = lockDir.resolve(lockFileName);
}
@Override
public boolean obtain() throws IOException {
try {
Files.createDirectories(lockDir);
Files.createFile(lockFile);
return true;
} catch (IOException ioe) {
// On Windows, on concurrent createNewFile, the 2nd process gets "access denied".
// In that case, the lock was not aquired successfully, so return false.
// We record the failure reason here; the obtain with timeout (usually the
// one calling us) will use this as "root cause" if it fails to get the lock.
failureReason = ioe;
return false;
}
}
@Override
public void close() throws LockReleaseFailedException {
// TODO: wierd that clearLock() throws the raw IOException...
try {
Files.deleteIfExists(lockFile);
} catch (Throwable cause) {
throw new LockReleaseFailedException("failed to delete " + lockFile, cause);
}
}
@Override
public boolean isLocked() {
return Files.exists(lockFile);
}
@Override
public String toString() {
return "[email protected]" + lockFile;
}
}
}
在SimpleFSLockFactory定义了一个内部类SimpleFSLock继承Lock，我们还是主要看SimpleFSLockFactory的obtain方法，这里就是SimpleFSLock具体实现文件锁的代码。
Files.createDirectories(lockDir); Files.createFile(lockFile);
可以看着两行代码，createDirectories建立write.lock（可以是别的文件名，lucene默认使用write.lock）文件所在的文件夹及父文件夹。createFile则是创建write.lock文件，这里有一个精妙的地方，如果write.lock已经存在，那么createFile则会抛出异常，如果抛出异常，则表明SimpleFSLockFactory维持文件锁失败，也即意味着别的进程正在写索引文件。
看到close()方法中Files.deleteIfExists(lockFile); 就表示如果每次关闭IndexWriter，则会删除write.lock文件。
总结一下，SimpleFSLockFactory加文件锁的机制可以通俗的理解为，在索引文件所在的目录下，创建一个write.lock文件，如果此文件夹下已经有write.lock文件，则表明已经有其他进程在写当前的索引目录，所以此次添加文件锁失败，也即不能像索引文件中添加信息。每次添加完信息后，则会删除write.lock文件，释放文件锁。也即如果write.lock文件存在，就表明已经有进程在写索引文件，如果write.lock不存在就创建文件并添加了文件锁，别的进程不能写文件。
这是一个非常精妙的方式去实现写文件排他锁，当然可能有些读者会疑惑为什么自己在Demo中，创建完索引，close后还有write.lock文件存在，因为现在lucene的默认实现是NativeFSLockFactory，也是上文提及的使用nio调用本地方法去实现的lock。