原理

quartz用的是行锁,for update 是排他锁,一旦获取TRIGGER_ACCESS行锁,这时候针对这行读写都不能进行

session 1

set autocommit=0;
select * from QRTZ_LOCKS where lock_name = 'TRIGGER_ACCESS' for update ;

session2

session2将会被锁住

select * from QRTZ_LOCKS where lock_name = 'TRIGGER_ACCESS' for update ;
//等待

代码实现

自定义SemaPhore

public interface Semaphore {
    /*
     * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     * 
     * Interface.
     * 
     * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     */

    /**
     * Grants a lock on the identified resource to the calling thread (blocking
     * until it is available).
     * 
     * @param conn Database connection used to establish lock.  Can be null if
     * <code>{@link #requiresConnection()}</code> returns false.
     * 
     * @return true if the lock was obtained.
   
     */
    boolean obtainLock(Connection conn, String lockName) throws LockException;

    /**
     * Release the lock on the identified resource if it is held by the calling
     * thread.
     */
    void releaseLock(String lockName) throws LockException;

    /**
     * Whether this Semaphore implementation requires a database connection for
     * its lock management operations.
     * 
     * @see #obtainLock(Connection, String)
     * @see #releaseLock(String)
     */
    boolean requiresConnection();
}

实现者有DBSemaphore,SimpleSemaphore,JTANonClusteredSemaphore

obtainLock,是获取锁

对于DBSemaphore是获取数据库表的行的排它锁.

 public synchronized boolean obtainLock(Connection conn, String lockName) {

        lockName = lockName.intern();

        if(log.isDebugEnabled()) {
            log.debug(
                "Lock '" + lockName + "' is desired by: "
                        + Thread.currentThread().getName());
        }

        if (!isLockOwner(lockName)) {
            if(log.isDebugEnabled()) {
                log.debug(
                    "Lock '" + lockName + "' is being obtained: "
                            + Thread.currentThread().getName());
            }
            while (locks.contains(lockName)) {
                try {
                    this.wait();
                } catch (InterruptedException ie) {
                    if(log.isDebugEnabled()) {
                        log.debug(
                            "Lock '" + lockName + "' was not obtained by: "
                                    + Thread.currentThread().getName());
                    }
                }
            }

            if(log.isDebugEnabled()) {
                log.debug(
                    "Lock '" + lockName + "' given to: "
                            + Thread.currentThread().getName());
            }
            getThreadLocks().add(lockName);
            locks.add(lockName);
        } else if(log.isDebugEnabled()) {
            log.debug(
                "Lock '" + lockName + "' already owned by: "
                        + Thread.currentThread().getName()
                        + " -- but not owner!",
                new Exception("stack-trace of wrongful returner"));
        }

        return true;
    }