Spring实现数据库读写分离

转载自:http://itindex.net/detail/50093-spring-%E6%95%B0%E6%8D%AE%E5%BA%93
              http://blog.csdn.net/alaahong/article/details/8707915

       在数据库层面需要采用读写分离技术,就是一个master数据库,多个slave数据库。master库负责数据更新和实时数据查询,slave库负责非实时数据查询。因为在实际的应用中,数据库都是读多写少(读取数据的频率高,更新数据的频率相对较少),而读取数据通常耗时比较长,占用数据库服务器的CPU较多。

       采用读写分离技术的目标:有效减轻master库的压力,又可以把用户查询数据的请求分发到不同的slave库,从而保证系统的健壮性。


       如何方便的实现读写分离呢?
       1.第一种方式是最简单的方式,就是定义2个数据库连接,一个是masterDataSource,另一个是slaveDataSource。对DAO的dataSource属性注入是,根据需求分别如入不同的DataSource。有时由于在同一个DAO中可能既有select又有insert,那样就需要对同一类型的DAO写两个DAO,比如就可能需要将UserDaoImp拆分为UserDaoImp_w与UserDaoImp_r。
       2. 第二种方式,不将UserDaoImp拆分为UserDaoImp_w与UserDaoImp_r。在UserDaoImp的方法调用前使用this.setDataSource()来切换DataSource,但是存在线程安全问题。例如,线程一中,调用了f1(),将datasource切换为masterDataSource ,准备写数据到主数据库,但是这时线程二,调用了f2(),将datasource切换为slaveDataSource,准备从从数据库读数据,那么线程一继续执行f1()的方法就会出现问题。原因在于线程一、线程二使用的是一个UserDaoImp实例(spring加载的bean默认都是单例),共用的一个datasource。当然可不让UserDaoImp为单例,并结合ThreadLocal来避免线程安全问题,但是不推荐。
       3. 第三种方式动态数据源切换,就是在程序运行时,把数据源动态织入到程序中,从而选择读取主库还是从库。 这里Spring的AbstractRoutingDataSource提供了很好的支持。
public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean {

	private Map<Object, Object> targetDataSources;

	private Object defaultTargetDataSource;

	private boolean lenientFallback = true;

	private DataSourceLookup dataSourceLookup = new JndiDataSourceLookup();

	private Map<Object, DataSource> resolvedDataSources;

	private DataSource resolvedDefaultDataSource;
        
        ......
       AbstractRoutingDataSource继承了AbstractDataSource ,而AbstractDataSource 又是DataSource 的子类。DataSource是javax.sql的数据源接口,定义如下:
public interface DataSource  extends CommonDataSource, Wrapper {

  Connection getConnection() throws SQLException;
 
  Connection getConnection(String username, String password)
    throws SQLException;
}
       DataSource 接口定义了2个方法,都是获取数据库连接。在来看下AbstractRoutingDataSource 如何实现了DataSource接口:
public Connection getConnection() throws SQLException {
	return determineTargetDataSource().getConnection();
}

public Connection getConnection(String username, String password) throws SQLException {
	return determineTargetDataSource().getConnection(username, password);
}
       AbstractRoutingDataSource通过调用determineTargetDataSource()方法获取到connection。determineTargetDataSource方法定义如下:
protected DataSource determineTargetDataSource() {
	Assert.notNull(this.resolvedDataSources, "DataSource router not initialized");
	Object lookupKey = determineCurrentLookupKey();
	DataSource dataSource = this.resolvedDataSources.get(lookupKey);
	if (dataSource == null && (this.lenientFallback || lookupKey == null)) {
		dataSource = this.resolvedDefaultDataSource;
	}
	if (dataSource == null) {
		throw new IllegalStateException("Cannot determine target DataSource for lookup key [" + lookupKey + "]");
	}
	return dataSource;
}
      determineCurrentLookupKey()方法返回lookupKey, resolvedDataSources()方法就是根据 lookupKey从Map中获得数据源。

示例:
beans.xml
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="
    http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">

	<bean id="m_dataSource"
		class="org.springframework.jdbc.datasource.DriverManagerDataSource">
		<property name="driverClassName" value="com.mysql.jdbc.Driver" />
		<property name="url" value="jdbc:mysql://127.0.0.1:3306/zero" />
		<property name="username" value="root/>
		<property name="password" value="123456" />
	</bean>

	<bean id="s_dataSource"
		class="org.springframework.jdbc.datasource.DriverManagerDataSource">
		<property name="driverClassName" value="com.mysql.jdbc.Driver" />
		<property name="url"
			value="jdbc:mysql://127.0.0.1:3306/zero_test" />
		<property name="username" value="root" />
		<property name="password" value="123456" />
	</bean>

	<bean id="dynamicDataSource" class="com.zero.springjdbc.DynamicDataSource">
		<property name="targetDataSources">
			<map key-type="java.lang.String">
				<!-- write -->
				<entry key="master" value-ref="m_dataSource" />
				<!-- read -->
				<entry key="slave" value-ref="s_dataSource" />
			</map>
		</property>
		<property name="defaultTargetDataSource" ref="m_dataSource" />
	</bean>


	<bean id="zeroDaoImpl" class="com.zero.springjdbc.ZeroDaoImpl">
		<property name="dataSource" ref="dynamicDataSource"></property>
	</bean>

</beans>
       以上的beans.xml中,配置了两个DataSource,m_dataSource与s_dataSource,然后将这两个DataSource交给com.zero.springjdbc.DynamicDataSource管理,由于com.zero.springjdbc.DynamicDataSource是继承了AbstractRoutingDataSource,所以给它的两个属性targetDataSources、defaultTargetDataSource注入值。而DAO层的dataSource属性所使用的是dynamicDataSource。

DynamicDataSourceHolder.java
package com.zero.springjdbc;

public class DynamicDataSourceHolder {
	public static final ThreadLocal<String> holder = new ThreadLocal<String>();

	public static void setDataSource(String name) {
		holder.set(name);
	}

	public static String getDataSouce() {
		return holder.get();
	}
}
       这里利用了ThreadLocal,来指明每个线程在进行数据库操作时所用到的数据库。

DynamicDataSource.java
package com.zero.springjdbc;

import java.sql.SQLFeatureNotSupportedException;
import java.util.logging.Logger;

import org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource;

public class DynamicDataSource extends AbstractRoutingDataSource{

	@Override
	protected Object determineCurrentLookupKey() {
		// TODO Auto-generated method stub
		return DynamicDataSourceHolder.getDataSouce();
	}

	@Override
	public Logger getParentLogger() throws SQLFeatureNotSupportedException {
		// TODO Auto-generated method stub
		return null;
	}
}
ZeroDaoImpl.java
package com.zero.springjdbc;

import org.springframework.jdbc.core.JdbcTemplate;

public class ZeroDaoImpl extends JdbcTemplate {

	public void update1() {
		DynamicDataSourceHolder.setDataSource("master");
		String sql01 = "update lineitem_record set fee=111111 where lineitemId='11238'";
		System.out.println("update1 lineitemId='11238' : "
				+ this.update(sql01));
		
		String sql02 = "update lineitem_record set fee=111111 where lineitemId='111'";
		System.out.println("update1 lineitemId='111' : "
				+ this.update(sql02));
	}

	public void update2() {
		try {
			
			DynamicDataSourceHolder.setDataSource("slave");
			String sql01 = "update lineitem_record set fee=111111 where lineitemId='11238'";
			System.out.println("update2 lineitemId='11238' : "
					+ this.update(sql01));
			
			String sql02 = "update lineitem_record set fee=222222 where lineitemId='111'";
			System.out.println("update2 lineitemId='111' : "
					+ this.update(sql02));
		} catch (Exception e) {
			// TODO: handle exception
			e.printStackTrace();
		}
	}
}
       这里的每个方法都DynamicDataSourceHolder.setDataSource(),来指明接下来的操作将使用的是什么数据库,由于被ThreadLocal隔离了,所以是线程安全的。

Test.java
package com.zero.springjdbc;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		ApplicationContext ctx = new ClassPathXmlApplicationContext(
				"/com/zero/springjdbc/beans.xml");
		ZeroDaoImpl zeroDaoImpl = (ZeroDaoImpl) ctx.getBean("zeroDaoImpl");
		for (int i = 0; i < 5; i++) {
			new Thread(new Runnable() {

				@Override
				public void run() {
					// TODO Auto-generated method stub
					zeroDaoImpl.update1();
				}
			}, "thread1-" + i).start();
			;
			new Thread(new Runnable() {

				@Override
				public void run() {
					// TODO Auto-generated method stub
					zeroDaoImpl.update2();
				}
			}, "thread2-" + i).start();
			;
		}

	}

}
      这里建了10个线程,每个线程执行zeroDaoImpl.update1()或zeroDaoImpl.update2()方法来检验是否是线程安全的。
      PS:master的lineitem_record表中有lineitemId='11238',没有lineitemId='111';而slave的lineitem_record表中没有lineitemId='11238',有lineitemId='111'。预期结果是update1 lineitemId='11238' 的值是1,而update1 lineitemId='111'的值是0,update2 lineitemId='11238' 的值是0,而update2 lineitemId='111'的值是1。如果不是这样,那么就说明在多线程环境下,依然会出现方式二所面临的问题。
测试结果:
update2 lineitemId='11238' : 0
update1 lineitemId='11238' : 1
update2 lineitemId='11238' : 0
update1 lineitemId='11238' : 1
update2 lineitemId='11238' : 0
update1 lineitemId='11238' : 1
update1 lineitemId='11238' : 1
update1 lineitemId='11238' : 1
update2 lineitemId='11238' : 0
update1 lineitemId='111' : 0
update2 lineitemId='111' : 1
update1 lineitemId='111' : 0
update1 lineitemId='111' : 0
update2 lineitemId='111' : 1
update2 lineitemId='111' : 1
update1 lineitemId='111' : 0
update2 lineitemId='11238' : 0
update2 lineitemId='111' : 1
update1 lineitemId='111' : 0
update2 lineitemId='111' : 1
      由结果可知, 利用Spring的AbstractRoutingDataSource可以解决多数据源的问题

贴上AbstractRoutingDataSource源码:
package org.springframework.jdbc.datasource.lookup;

import java.sql.Connection;
import java.sql.SQLException;
import java.util.HashMap;
import java.util.Map;
import javax.sql.DataSource;

import org.springframework.beans.factory.InitializingBean;
import org.springframework.jdbc.datasource.AbstractDataSource;
import org.springframework.util.Assert;

public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean {

	private Map<Object, Object> targetDataSources;

	private Object defaultTargetDataSource;

	private boolean lenientFallback = true;

	private DataSourceLookup dataSourceLookup = new JndiDataSourceLookup();

	private Map<Object, DataSource> resolvedDataSources;

	private DataSource resolvedDefaultDataSource;

	public void setTargetDataSources(Map<Object, Object> targetDataSources) {
		this.targetDataSources = targetDataSources;
	}

	public void setDefaultTargetDataSource(Object defaultTargetDataSource) {
		this.defaultTargetDataSource = defaultTargetDataSource;
	}

	public void setLenientFallback(boolean lenientFallback) {
		this.lenientFallback = lenientFallback;
	}

	public void setDataSourceLookup(DataSourceLookup dataSourceLookup) {
		this.dataSourceLookup = (dataSourceLookup != null ? dataSourceLookup : new JndiDataSourceLookup());
	}


	public void afterPropertiesSet() {
		if (this.targetDataSources == null) {
			throw new IllegalArgumentException("Property 'targetDataSources' is required");
		}
		this.resolvedDataSources = new HashMap<Object, DataSource>(this.targetDataSources.size());
		for (Map.Entry entry : this.targetDataSources.entrySet()) {
			Object lookupKey = resolveSpecifiedLookupKey(entry.getKey());
			DataSource dataSource = resolveSpecifiedDataSource(entry.getValue());
			this.resolvedDataSources.put(lookupKey, dataSource);
		}
		if (this.defaultTargetDataSource != null) {
			this.resolvedDefaultDataSource = resolveSpecifiedDataSource(this.defaultTargetDataSource);
		}
	}

	protected Object resolveSpecifiedLookupKey(Object lookupKey) {
		return lookupKey;
	}

	protected DataSource resolveSpecifiedDataSource(Object dataSource) throws IllegalArgumentException {
		if (dataSource instanceof DataSource) {
			return (DataSource) dataSource;
		}
		else if (dataSource instanceof String) {
			return this.dataSourceLookup.getDataSource((String) dataSource);
		}
		else {
			throw new IllegalArgumentException(
					"Illegal data source value - only [javax.sql.DataSource] and String supported: " + dataSource);
		}
	}


	public Connection getConnection() throws SQLException {
		return determineTargetDataSource().getConnection();
	}

	public Connection getConnection(String username, String password) throws SQLException {
		return determineTargetDataSource().getConnection(username, password);
	}

	@Override
	@SuppressWarnings("unchecked")
	public <T> T unwrap(Class<T> iface) throws SQLException {
		if (iface.isInstance(this)) {
			return (T) this;
		}
		return determineTargetDataSource().unwrap(iface);
	}

	@Override
	public boolean isWrapperFor(Class<?> iface) throws SQLException {
		return (iface.isInstance(this) || determineTargetDataSource().isWrapperFor(iface));
	}

	protected DataSource determineTargetDataSource() {
		Assert.notNull(this.resolvedDataSources, "DataSource router not initialized");
		Object lookupKey = determineCurrentLookupKey();
		DataSource dataSource = this.resolvedDataSources.get(lookupKey);
		if (dataSource == null && (this.lenientFallback || lookupKey == null)) {
			dataSource = this.resolvedDefaultDataSource;
		}
		if (dataSource == null) {
			throw new IllegalStateException("Cannot determine target DataSource for lookup key [" + lookupKey + "]");
		}
		return dataSource;
	}

	protected abstract Object determineCurrentLookupKey();

}








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