trunk/camel-core/src/main/java/org/apache/camel/component/file/strategy/GenericFileRenameExclusiveReadLockStrategy.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.camel.component.file.strategy;

import org.apache.camel.Exchange;
import org.apache.camel.component.file.GenericFile;
import org.apache.camel.component.file.GenericFileEndpoint;
import org.apache.camel.component.file.GenericFileExclusiveReadLockStrategy;
import org.apache.camel.component.file.GenericFileOperations;
import org.apache.camel.util.StopWatch;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Acquires exclusive read lock to the given file. Will wait until the lock is granted.
 * After granting the read lock it is released, we just want to make sure that when we start
 * consuming the file its not currently in progress of being written by third party.
 */
public class GenericFileRenameExclusiveReadLockStrategy<T> implements GenericFileExclusiveReadLockStrategy<T> {
    private static final transient Logger LOG = LoggerFactory.getLogger(GenericFileRenameExclusiveReadLockStrategy.class);
    private long timeout;
    private long checkInterval;

    public void prepareOnStartup(GenericFileOperations<T> operations, GenericFileEndpoint<T> endpoint) throws Exception {
        // noop
    }

    public boolean acquireExclusiveReadLock(GenericFileOperations<T> operations, GenericFile<T> file,
                                            Exchange exchange) throws Exception {
        LOG.trace("Waiting for exclusive read lock to file: {}", file);

        // the trick is to try to rename the file, if we can rename then we have exclusive read
        // since its a Generic file we cannot use java.nio to get a RW lock
        String newName = file.getFileName() + ".camelExclusiveReadLock";

        // make a copy as result and change its file name
        GenericFile<T> newFile = file.copyFrom(file);
        newFile.changeFileName(newName);
        StopWatch watch = new StopWatch();

        boolean exclusive = false;
        while (!exclusive) {
            // timeout check
            if (timeout > 0) {
                long delta = watch.taken();
                if (delta > timeout) {
                    LOG.warn("Cannot acquire read lock within " + timeout + " millis. Will skip the file: " + file);
                    // we could not get the lock within the timeout period, so return false
                    return false;
                }
            }

            exclusive = operations.renameFile(file.getAbsoluteFilePath(), newFile.getAbsoluteFilePath());
            if (exclusive) {
                LOG.trace("Acquired exclusive read lock to file: {}", file);
                // rename it back so we can read it
                operations.renameFile(newFile.getAbsoluteFilePath(), file.getAbsoluteFilePath());
            } else {
                boolean interrupted = sleep();
                if (interrupted) {
                    // we were interrupted while sleeping, we are likely being shutdown so return false
                    return false;
                }
            }
        }

        return true;
    }

    public void releaseExclusiveReadLock(GenericFileOperations<T> operations, GenericFile<T> file,
                                         Exchange exchange) throws Exception {
        // noop
    }

    private boolean sleep() {
        LOG.trace("Exclusive read lock not granted. Sleeping for {} millis.", checkInterval);
        try {
            Thread.sleep(checkInterval);
            return false;
        } catch (InterruptedException e) {
            LOG.debug("Sleep interrupted while waiting for exclusive read lock, so breaking out");
            return true;
        }
    }

    public long getTimeout() {
        return timeout;
    }

    public void setTimeout(long timeout) {
        this.timeout = timeout;
    }

    public void setCheckInterval(long checkInterval) {
        this.checkInterval = checkInterval;
    }
}