ActiveMQ_032：共享文件存储方式的锁机制

环境：MAC OS X 10.11.5 + ActiveMQ 5.13.3

核心代码是 SharedFileLocker.java 和 LockFile.java，核心方法是 doStart 和 keepAlive。
其原理就是 java 文件锁，只要 ActiveMQ 进程在就锁住该文件，没有类似心跳或每隔一段时间更新 lock 时间的机制。

1. SharedFileLocker.java

package org.apache.activemq.store;

import java.io.File;
import java.io.IOException;
import java.util.concurrent.TimeUnit;

import org.apache.activemq.broker.AbstractLocker;
import org.apache.activemq.util.LockFile;
import org.apache.activemq.util.ServiceStopper;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Represents an exclusive lock on a database to avoid multiple brokers running
 * against the same logical database.
 *
 * @org.apache.xbean.XBean element="shared-file-locker"
 *
 */
public class SharedFileLocker extends AbstractLocker {

    public static final File DEFAULT_DIRECTORY = new File("KahaDB");
    private static final Logger LOG = LoggerFactory.getLogger(SharedFileLocker.class);

    private LockFile lockFile;
    protected File directory = DEFAULT_DIRECTORY;

    @Override
    public void doStart() throws Exception {
        if (lockFile == null) {
            File lockFileName = new File(directory, "lock");
            lockFile = new LockFile(lockFileName, false);
            if (failIfLocked) {
                lockFile.lock();
            } else {
                // Print a warning only once
                boolean warned = false;
                boolean locked = false;
                while ((!isStopped()) && (!isStopping())) {
                    try {
                        lockFile.lock();
                        if (warned) {
                            // ensure lockHolder has released; wait for one keepAlive iteration
                            try {
                                TimeUnit.MILLISECONDS.sleep(lockable != null ? lockable.getLockKeepAlivePeriod() : 0l);
                            } catch (InterruptedException e1) {
                            }
                        }
                        locked = keepAlive();
                        break;
                    } catch (IOException e) {
                        if (!warned) {
                            LOG.info("Database "
                                    + lockFileName
                                    + " is locked by another server. This broker is now in slave mode waiting a lock to be acquired");
                            warned = true;
                        }

                        LOG.debug("Database "
                                + lockFileName
                                + " is locked... waiting "
                                + (lockAcquireSleepInterval / 1000)
                                + " seconds for the database to be unlocked. Reason: "
                                + e);
                        try {
                            TimeUnit.MILLISECONDS.sleep(lockAcquireSleepInterval);
                        } catch (InterruptedException e1) {
                        }
                    }
                }
                if (!locked) {
                    throw new IOException("attempt to obtain lock aborted due to shutdown");
                }
            }
        }
    }

    @Override
    public boolean keepAlive() {
        boolean result = lockFile != null && lockFile.keepAlive();
        LOG.trace("keepAlive result: " + result + (name != null ? ", name: " + name : ""));
        return result;
    }

    @Override
    public void doStop(ServiceStopper stopper) throws Exception {
        if (lockFile != null) {
            lockFile.unlock();
            lockFile = null;
        }
    }

    public File getDirectory() {
        return directory;
    }

    public void setDirectory(File directory) {
        this.directory = directory;
    }

    @Override
    public void configure(PersistenceAdapter persistenceAdapter) throws IOException {
        this.setDirectory(persistenceAdapter.getDirectory());
        if (name == null) {
            name = getDirectory().toString();
        }
    }
}

2. LockFile.java

package org.apache.activemq.util;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.channels.FileLock;
import java.nio.channels.OverlappingFileLockException;
import java.util.Date;

/**
 * Used to lock a File.
 *
 * @author chirino
 */
public class LockFile {

    private static final boolean DISABLE_FILE_LOCK = Boolean.getBoolean("java.nio.channels.FileLock.broken");
    final private File file;
    private long lastModified;

    private FileLock lock;
    private RandomAccessFile randomAccessLockFile;
    private int lockCounter;
    private final boolean deleteOnUnlock;
    private volatile boolean locked;
    private String lockSystemPropertyName = "";

    private static final Logger LOG = LoggerFactory.getLogger(LockFile.class);

    public LockFile(File file, boolean deleteOnUnlock) {
        this.file = file;
        this.deleteOnUnlock = deleteOnUnlock;
    }

    /**
     * @throws IOException
     */
    synchronized public void lock() throws IOException {
        if (DISABLE_FILE_LOCK) {
            return;
        }

        if (lockCounter > 0) {
            return;
        }

        IOHelper.mkdirs(file.getParentFile());
        synchronized (LockFile.class) {
            lockSystemPropertyName = getVmLockKey();
            if (System.getProperty(lockSystemPropertyName) != null) {
                throw new IOException("File '" + file + "' could not be locked as lock is already held for this jvm. Value: " + System.getProperty(lockSystemPropertyName));
            }
            System.setProperty(lockSystemPropertyName, new Date().toString());
        }
        try {
            if (lock == null) {
                randomAccessLockFile = new RandomAccessFile(file, "rw");
                IOException reason = null;
                try {
                    lock = randomAccessLockFile.getChannel().tryLock(0, Math.max(1, randomAccessLockFile.getChannel().size()), false);
                } catch (OverlappingFileLockException e) {
                    reason = IOExceptionSupport.create("File '" + file + "' could not be locked.", e);
                } catch (IOException ioe) {
                    reason = ioe;
                }
                if (lock != null) {
                    //track lastModified only if we are able to successfully obtain the lock.
                    randomAccessLockFile.writeLong(System.currentTimeMillis());
                    randomAccessLockFile.getChannel().force(true);
                    lastModified = file.lastModified();
                    lockCounter++;
                    System.setProperty(lockSystemPropertyName, new Date().toString());
                    locked = true;
                } else {
                    // new read file for next attempt
                    closeReadFile();
                    if (reason != null) {
                        throw reason;
                    }
                    throw new IOException("File '" + file + "' could not be locked.");
                }

            }
        } finally {
            synchronized (LockFile.class) {
                if (lock == null) {
                    System.getProperties().remove(lockSystemPropertyName);
                }
            }
        }
    }

    /**
     */
    synchronized public void unlock() {
        if (DISABLE_FILE_LOCK) {
            return;
        }

        lockCounter--;
        if (lockCounter != 0) {
            return;
        }

        // release the lock..
        if (lock != null) {
            try {
                lock.release();
            } catch (Throwable ignore) {
            } finally {
                if (lockSystemPropertyName != null) {
                    System.getProperties().remove(lockSystemPropertyName);
                }
                lock = null;
            }
        }
        closeReadFile();

        if (locked && deleteOnUnlock) {
            file.delete();
        }
    }

    private String getVmLockKey() throws IOException {
        return getClass().getName() + ".lock." + file.getCanonicalPath();
    }

    private void closeReadFile() {
        // close the file.
        if (randomAccessLockFile != null) {
            try {
                randomAccessLockFile.close();
            } catch (Throwable ignore) {
            }
            randomAccessLockFile = null;
        }
    }

    /**
     * @return true if the lock file's last modified does not match the locally
     * cached lastModified, false otherwise
     */
    private boolean hasBeenModified() {
        boolean modified = false;

        //Create a new instance of the File object so we can get the most up to date information on the file.
        File localFile = new File(file.getAbsolutePath());

        if (localFile.exists()) {
            if (localFile.lastModified() != lastModified) {
                LOG.info("Lock file " + file.getAbsolutePath() + ", locked at " + new Date(lastModified) + ", has been modified at " + new Date(localFile.lastModified()));
                modified = true;
            }
        } else {
            //The lock file is missing
            LOG.info("Lock file " + file.getAbsolutePath() + ", does not exist");
            modified = true;
        }

        return modified;
    }

    public boolean keepAlive() {
        locked = locked && lock != null && lock.isValid() && !hasBeenModified();
        return locked;
    }

}

参考文献：
1. https://github.com/apache/activemq/blob/master/activemq-broker/src/main/java/org/apache/activemq/store/SharedFileLocker.java
2. https://github.com/apache/activemq/blob/master/activemq-broker/src/main/java/org/apache/activemq/util/LockFile.java