Quartz (三) 锁机制

前言：一个Quartz集群中的每个节点是一个独立的Quartz应用，它又管理着其他的节点。这就意味着你必须对每个节点分别启动或停止。Quartz集群中，独立的Quartz节点并不与另一其的节点或是管理节点通信，而是通过相同的数据库表来感知到另一Quartz应用的

quartz 通过数据库的行级锁，来实现分布式锁（一种悲观锁）

通过这个机制，集群中一次只能有一个线程来操作加锁 - 操作 - 释放锁。来保证多个结点的应用只如果操作的时间过长的话，会带来集群间的主线程等待。
数据库行锁是一种悲观锁，锁表时其它线程无法查询。

Semaphore 接口

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 * All content copyright Terracotta, Inc., unless otherwise indicated. All rights reserved.
 *
 * Licensed 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
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 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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package org.quartz.impl.jdbcjobstore;

import java.sql.Connection;

/**
 * An interface for providing thread/resource locking in order to protect
 * resources from being altered by multiple threads at the same time.
 * 
 * @author jhouse
 */
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();
}

StdRowLockSemaphore.java

public static final String SELECT_FOR_LOCK = "SELECT * FROM "
            + TABLE_PREFIX_SUBST + TABLE_LOCKS + " WHERE " + COL_SCHEDULER_NAME + " = " + SCHED_NAME_SUBST
            + " AND " + COL_LOCK_NAME + " = ? FOR UPDATE";

    public static final String INSERT_LOCK = "INSERT INTO "
        + TABLE_PREFIX_SUBST + TABLE_LOCKS + "(" + COL_SCHEDULER_NAME + ", " + COL_LOCK_NAME + ") VALUES (" 
        + SCHED_NAME_SUBST + ", ?)";

使用 select ... for update 对数据库表进row进行加锁

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