It is possible to have multiple instances of a poller endpoint attempting to read a file on the system. To ensure that there are no conflicts in accessing the file, poller endpoints obtain an exclusive lock on a file while it is being processed.

The locking behavior is controlled by an implementation of the org.apache.servicemix.common.locks.LockManager interface. By default, poller endpoints use a provided implementation of this interface. If the default behavior is not appropriate for your application, you can implement the LockManager interface and configure your endpoints to use your implementation.

To implement a custom lock manager, you need to provide your own implementation of the org.apache.servicemix.common.locks.LockManager interface. The LockManager has single method, getLock() that needs to be implemented. Example 11 shows the signature for getLock().

The getLock() method takes a string that represents the URI of the file being processes and it returns a java.util.concurrent.locks.Lock object. The returned Lock object holds the lock for the specified file.

Example 12 shows a simple lock manager implementation.

package org.apache.servicemix.demo;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

import org.apache.servicemix.common.locks.LockManager;

public class myLockManager implements LockManager
{
  private ConcurrentMap<String, Lock> locks = new ConcurrentHashMap<String, Lock>();

  public Lock getLock(String id)
  {
    Lock lock = locks.get(id);
    if (lock == null)
    {
      lock = new ReentrantLock();
      Lock oldLock = locks.putIfAbsent(id, lock);
      if (oldLock != null)
      {
        lock = oldLock;
      }
    }
    return lock;
  }

}

You configure a poller endpoint to use a custom lock manager using its lockManager attribute. The lockManager attribute's value is a reference to a bean element specifying the class of your custom lock manager implementation.

Example 13 shows configuration for a poller endpoint that uses a custom lock manager.

<beans xmlns:file="http://servicemix.apache.org/file/1.0"
	       xmlns:foo="http://servicemix.org/demo/">

  <file:poller service="foo:filePoller"
               endpoint="filePoller"
               targetService="foo:fileSender" 
               file="inbox"
               lockManager="#myLockManager" />

  <bean id="myLockManager" class="org.apache.servicemix.demo.myLockManager" />
  ...
</beans>