大家都知道，在多線程或多進程的環境中，對統一資源的訪問需要特別小心，特別是在寫資源時，如果不加鎖，將會導致很多嚴重的后果，Lucene的索引也是如此，lucene對索引的讀寫分為IndexReader和IndexWriter，顧名思義，一個讀，一個寫，lucene可以對同一個索引文件建立多個IndexReader對象，但是只能有一個IndexWriter對象，這是怎么做到的呢？顯而易見是需要加鎖的，加鎖可以保證一個索引文件只能建立一個IndexWriter對象。下面就細說Lucene索引文件鎖機制：
如果我們對同一個索引文件建立多個不同的IndexWriter會怎么樣呢？
IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer); IndexWriter indexWriter = new IndexWriter(dir, indexWriterConfig); IndexWriterConfig indexWriterConfig2 = new IndexWriterConfig(analyzer); IndexWriter indexWriter2 = new IndexWriter(dir,indexWriterConfig2);
運行后，控制台輸出：
Exception in thread "main" org.apache.lucene.store.LockObtainFailedException: Lock obtain timed out: NativeFSLock@C:\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)
顯然是不可以對同一個索引文件開啟多個IndexWriter。
上面是一個比較簡略的類圖，可以看到lucene采用了工廠方法，這樣可以方便擴展其他實現，這里只以SimpleFsLock為例說明lucene的鎖機制（其他的有興趣可以看lucene源碼）。
Lock類是鎖的基類，一個抽象類，源碼如下：
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()，這個方法用來維持鎖，建立鎖之后，維持時間為LOCK_POLL_INTERVAL，之后需要重新申請維持鎖，這樣做是為了支持多線程讀寫。當然也可以將lockWaitTimeout設置為-1，這樣就是一直維持寫鎖。
抽象基類LockFactory，只定義了一個抽象方法makeLock，返回Lock對象的一個實例。
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); }
抽象類FSLockFactory繼承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;
}
這個方法默認返回NativeFSLockFactory，和SimpleFSLockFactory一樣是一個具體實現，NativeFSLockFactory使用的是nio中FileChannel.tryLock方法，這里不展開討論，有興趣的讀者可以去看jdk nio的源碼（好像現在oracle不提供FileChannel實現類的源碼了，需要去jvm里找）。
下面就是本篇文章的重頭戲，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 "SimpleFSLock@" + lockFile; } } }
在SimpleFSLockFactory定義了一個內部類SimpleFSLock繼承Lock，我們還是主要看SimpleFSLockFactory的obtain方法，這里就是SimpleFSLock具體實現文件鎖的代碼。
Files.createDirectories(lockDir);
Files.createFile(lockFile);
可以看着兩行代碼，createDirectories建立write.lock（可以是別的文件名，lucene默認使用write.lock）文件所在的文件夾及父文件夾。createFile則是創建write.lock文件，這里有一個精妙的地方，如果write.lock已經存在，那么createFile則會拋出異常，如果拋出異常，則表明SimpleFSLockFactory維持文件鎖失敗，也即意味着別的進程正在寫索引文件。
看到close()方法中Files.deleteIfExists(lockFile); 就表示如果每次關閉IndexWriter，則會刪除write.lock文件。
總結一下，SimpleFSLockFactory加文件鎖的機制可以通俗的理解為，在索引文件所在的目錄下，創建一個write.lock文件，如果此文件夾下已經有write.lock文件，則表明已經有其他進程在寫當前的索引目錄，所以此次添加文件鎖失敗，也即不能像索引文件中添加信息。每次添加完信息后，則會刪除write.lock文件，釋放文件鎖。也即如果write.lock文件存在，就表明已經有進程在寫索引文件，如果write.lock不存在就創建文件並添加了文件鎖，別的進程不能寫文件。
這是一個非常精妙的方式去實現寫文件鎖，當然可能有些讀者會疑惑為什么自己在Demo中，創建完索引，close后還有write.lock文件存在，因為現在lucene的默認實現是NativeFSLockFactory，也是上文提及的使用nio調用本地方法去實現的lock。