Typical use might look like:
+ * new Lock.With(directory.makeLock("my.lock")) {
+ * public Object doBody() {
+ * ... code to execute while locked ...
+ * }
+ * }.run();
+ *
+ *
+ * @see Directory#makeLock(String)
+ */
+public abstract class Lock {
+
+ /** 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.
+ * @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 boolean obtain(long lockWaitTimeout) throws LockObtainFailedException, 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;
+ }
+ LockObtainFailedException e = new LockObtainFailedException(reason);
+ if (failureReason != null) {
+ e.initCause(failureReason);
+ }
+ throw e;
+ }
+ try {
+ Thread.sleep(LOCK_POLL_INTERVAL);
+ } catch (InterruptedException ie) {
+ throw new ThreadInterruptedException(ie);
+ }
+ locked = obtain();
+ }
+ return locked;
+ }
+
+ /** Releases exclusive access. */
+ public abstract void release() 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 lock 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 LockObtainFailedException, IOException {
+ boolean locked = false;
+ try {
+ locked = lock.obtain(lockWaitTimeout);
+ return doBody();
+ } finally {
+ if (locked)
+ lock.release();
+ }
+ }
+ }
+
+}