Spring的JNDI数据源连接池配置示例及Spring对JNDI实现分析

 

 

在使用 Tomcat服务器 + SpringFramework 进行JavaEE项目的开发部署的时候可以在Tomcat的配置文件中进行JDBC数据源的配置，具体步骤如下（这里省略了工程的建立步骤）：

 

1） 添加如下代码到tomcat的conf目录下的server.xml中：

<Context>
    <Resource name="jdbc/demoDB" auth="Container"
    type="javax.sql.DataSource"
    driverClassName="com.mysql.jdbc.Driver"
    url="jdbc:mysql://localhost:3306/demo"
    username="root"
    password="123"
    maxActive="50"
    maxIdle="30"
    maxWait="10000" />
</Context>

 完成上述步骤数据源的连接池配置已经完成，但是为了提高项目的可移植性，最好将上述第二步的内容放入到工程的META-INF目录的context.xml中（这个文件需要自行建立）：

<?xml version="1.0" encoding="UTF-8"?>
<Context>
      <Resource name="jdbc/demoDB" auth="Container"
      type="javax.sql.DataSource"
      driverClassName="com.mysql.jdbc.Driver"
      url="jdbc:mysql://localhost:3306/demo"
      username="root"
      password="123"
      maxActive="50"
      maxIdle="30"
      maxWait="10000" />
</Context>

 

2）在spring的配置文件，如applicationContext.xml中配置配置如下内容：

<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
<beans>
    <bean id="dataSource" class="org.springframework.jndi.JndiObjectFactoryBean">
        <property name="jndiName">
            <value>java:comp/env/jdbc/demoDB</value>
        </property>
    </bean>
    <bean id="jdbcTemplate" class="org.springframework.jdbc.core.JdbcTemplate">
        <property name="dataSource">
            <ref bean="dataSource" />
        </property>
    </bean>

    <!-- 这里是自定义的数据库基础操作类 -->
    <bean id="sqlBaseDAO" class="demo.BaseDAOImpl">
        <property name="jdbcTemplate">
            <ref bean="jdbcTemplate" />
        </property>
    </bean>
</beans>

 

3）建立数据库基础操作类 BaseDAOImpl

    接口代码：

public interface BaseDAO {

    public List<Map<String, Object>> select(String sql);

    public void update(String how);

    public void insert(Object obj);

    public void insert(String sql);

    public void save(String sql);

    public void edit(String sql);

    public void execute(String sql, PreparedStatementCallback callback);

    public void delete(String sql);

    public void insertObjects(String[] sqls);

    public Connection getConnection() throws Exception;

}

 

   实现类代码：

public class BaseDAOImpl implements BaseDAO {
    private JdbcTemplate jdbcTemplate;

    public void setJdbcTemplate(JdbcTemplate jdbcTemplate){
        this.jdbcTemplate = jdbcTemplate;
    }

    public void insert(Object obj) {

    }

    public void insert(String sql) {
        jdbcTemplate.execute(sql);
    }

    public void insertObjects(String[] sqls) {
        jdbcTemplate.batchUpdate(sqls);
    }

    public List<Map<String, Object>> select(String sql) {
        return jdbcTemplate.queryForList(sql);
    }

    public void update(String how) {
        jdbcTemplate.update(how);

    }

    public void delete(String sql) {
        if (sql == null) {
            return;
        }
        jdbcTemplate.execute(sql);
    }

    public void edit(String sql) {
        if (sql == null) {
            return;
        }
        jdbcTemplate.execute(sql);
    }

    public void execute(String sql, PreparedStatementCallback callback) {
        jdbcTemplate.execute(sql, callback);
    }

    public void save(String sql) {
        if (sql == null) {
            return;
        }
        jdbcTemplate.execute(sql);
    }

    public Connection getConnection() throws Exception {
        Connection conn = jdbcTemplate.getDataSource().getConnection();
        return conn;
    }

}

 

 

这里存在一个疑问：

运行如下代码：

public static void main(String[] args) {
    org.springframework.jndi.JndiObjectFactoryBean jofb = new org.springframework.jndi.JndiObjectFactoryBean();
    javax.sql.DataSource ds = (javax.sql.DataSource)jofb;
    org.springframework.jdbc.core.JdbcTemplate jTemplate = new org.springframework.jdbc.core.JdbcTemplate();
    jTemplate.setDataSource(ds);
}

 

会报告如下的错误：

Exception in thread "main" java.lang.ClassCastException: org.springframework.jndi.JndiObjectFactoryBean cannot be cast to javax.sql.DataSource

从JndiObjectFactoryBean的源码中也可以看到，JndiObjectFactoryBean的父类或所继承的接口都没有继承javax.sql.DataSource接口，所以一下的配置中：

<bean id="dataSource" class="org.springframework.jndi.JndiObjectFactoryBean">
    <property name="jndiName">
        <value>java:comp/env/jdbc/portalDataService</value>
    </property>
</bean>
<bean id="jdbcTemplate" class="org.springframework.jdbc.core.JdbcTemplate">
    <property name="dataSource">
        <ref bean="dataSource" />
    </property>
</bean>

 对org.springframework.jdbc.core.JdbcTemplate的dataSource属性的注入为何能够成功？

 

带着这样的疑问去iteye中提问，没有得到详细的解答，但是iteye的提示功能似乎很不错，在问题的下方给出了相关内容参考提示，进入到《从源代码解读spring之DataSource实现和FactoryBean模式》这个帖子中，看完以后大受启发。一下是从这篇帖子摘抄出来的内容：

 

 

再看源码后发现，JndiObjectFactoryBean实现了FactoryBean接口，下面是org.springframework.beans.factory.FactoryBean源代码里一段注释： 

 

/**
 * Interface to be implemented by objects used within a BeanFactory
 * that are themselves factories. If a bean implements this interface,
 * it is used as a factory, not directly as a bean.
 *
 * <p><b>NB: A bean that implements this interface cannot be used
 * as a normal bean.</b> A FactoryBean is defined in a bean style,
 * but the object exposed for bean references is always the object
 * that it creates.
 */

 

翻译过来是说：所有实现FactoryBean接口的类都被当作工厂来使用，而不是简单的直接当作bean来使用，FactoryBean实现类里定义了要生产的对象，并且由FactoryBean实现类来造该对象的实例，看到这里聪明的大概已经能猜出个八九不离十了吧，我们回过头来看看JndiObjectFactoryBean的实现细节 ：

 

private Object jndiObject;
/**
 * Look up the JNDI object and store it.
 * 广义上说是造对象的过程，就本例而言，是通过JNDI获得DataSource对象
 */
public void afterPropertiesSet() throws IllegalArgumentException, NamingException {
    super.afterPropertiesSet();

    if (this.proxyInterface != null) {
        if (this.defaultObject != null) {
            throw new IllegalArgumentException(
                    "'defaultObject' is not supported in combination with 'proxyInterface'");
        }
        // We need a proxy and a JndiObjectTargetSource.
        this.jndiObject = JndiObjectProxyFactory.createJndiObjectProxy(this);
    }

    else {
        if (!this.lookupOnStartup || !this.cache) {
            throw new IllegalArgumentException(
                "Cannot deactivate 'lookupOnStartup' or 'cache' without specifying a 'proxyInterface'");
        }
        if (this.defaultObject != null && getExpectedType() != null &&
                !getExpectedType().isInstance(this.defaultObject)) {
            throw new IllegalArgumentException("Default object [" + this.defaultObject +
                    "] of type [" + this.defaultObject.getClass().getName() +
                    "] is not of expected type [" + getExpectedType().getName() + "]");
        }
        // Locate specified JNDI object.
        this.jndiObject = lookupWithFallback();
    }
}
/**
 * Return the singleton JNDI object.
 * 返回JNDI对象（DataSource对象）
 */
public Object getObject() {
    return this.jndiObject;
}

public Class getObjectType() {
    if (this.proxyInterface != null) {
        return this.proxyInterface;
    }
    else if (this.jndiObject != null) {
        return this.jndiObject.getClass();
    }
    else {
        return getExpectedType();
    }
}

 

对于JndiObjectFactoryBean对象，spring IOC容器启动时确实造了它的对象，只不过这时是工厂本身，spring会自动调用工厂里的afterPropertiesSet()方法去造真正需要的bean，然后调用getObject()和getObjectType()方法返回已造好的对象和类型，再将其准确的注入依赖它的其他bean里面。

 

 

好吧，也许上面org.springframework.beans.factory.FactoryBean的注释看起来像家长教育孩子该怎么怎么，那么Spring到底是怎么实现这种思想的呢？参考《Spring技术内幕》中2.5.3节对FactoryBean的实现的讲解，结合Spring的源码可以看到：

     常见的工厂Bean是怎样实现的，这些FactoryBean为应用生成需要的对象，这些对象往往是经过特殊处理的，比如像 ProxyFactoryBean 这样的特殊 Bean。FactoryBean 的生产特性是在getBean中起作用的，我们看到下面的调用：

再来看FactoryBean特性的实现：

 

//该方法在org.springframework.beans.factory.support.AbstractBeanFactory类中
protected Object getObjectForBeanInstance(
        Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {

    // Don't let calling code try to dereference the factory if the bean isn't a factory.
    // 如果这里不是对FactoryBean的调用,那么结束处理。
    if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
        throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
    }

    // Now we have the bean instance, which may be a normal bean or a FactoryBean.
    // If it's a FactoryBean, we use it to create a bean instance, unless the
    // caller actually wants a reference to the factory.
    if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
        return beanInstance;
    }

    Object object = null;
    if (mbd == null) {
        object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
        // Return bean instance from factory.
        FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
        // Caches object obtained from FactoryBean if it is a singleton.
        if (mbd == null && containsBeanDefinition(beanName)) {
            mbd = getMergedLocalBeanDefinition(beanName);
        }
        boolean synthetic = (mbd != null && mbd.isSynthetic());
        //这里从FactoryBean中得到bean。
        object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    return object;
}

//该方法在org.springframework.beans.factory.support.FactoryBeanRegistrySupport类中
protected Object getObjectFromFactoryBean(FactoryBean factory, String beanName, boolean shouldPostProcess) {
    if (factory.isSingleton() && containsSingleton(beanName)) {
        synchronized (getSingletonMutex()) {
            Object object = this.factoryBeanObjectCache.get(beanName);
            if (object == null) {
                object = doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
                this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
            }
            return (object != NULL_OBJECT ? object : null);
        }
    }
    else {
        return doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
    }
}

//该方法在org.springframework.beans.factory.support.FactoryBeanRegistrySupport类中
private Object doGetObjectFromFactoryBean(
        final FactoryBean factory, final String beanName, final boolean shouldPostProcess)
        throws BeanCreationException {

    Object object;
    //这里调用factory的getObject方法来从FactoryBean中得到bean。
    try {
        if (System.getSecurityManager() != null) {
            AccessControlContext acc = getAccessControlContext();
            try {
                object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                    public Object run() throws Exception {
                            return factory.getObject();
                        }
                    }, acc);
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            object = factory.getObject();
        }
    }
    catch (FactoryBeanNotInitializedException ex) {
        throw new BeanCurrentlyInCreationException(beanName, ex.toString());
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
    }

    // Do not accept a null value for a FactoryBean that's not fully
    // initialized yet: Many FactoryBeans just return null then.
    if (object == null && isSingletonCurrentlyInCreation(beanName)) {
        throw new BeanCurrentlyInCreationException(
                beanName, "FactoryBean which is currently in creation returned null from getObject");
    }

    if (object != null && shouldPostProcess) {
        try {
            object = postProcessObjectFromFactoryBean(object, beanName);
        }
        catch (Throwable ex) {
            throw new BeanCreationException(beanName, "Post-processing of the FactoryBean's object failed", ex);
        }
    }

    return object;
}

     这里返回的已经是作为工厂的 FactoryBean 生产的产品，并不是 FactoryBean 本身。这种FactoryBean的机制可以为我们提供一个很好的封装机制，比如封装Proxy、RMI、JNDI等。经过对FactoryBean实现过程的原理分析，相信读者会对getObject方法有很深刻的印象。这个方法就是主要的FactoryBean 的接口，需要实现特定的工厂的生产过程，至于这个生产过程是怎样和IoC容器整合的，就是我们在上面分析的内容。

 

那么返回的类型是怎么确定为javax.sql.DataSource类型的呢？回头再看在context.xml中的数据源配置可以看到：

 

type="javax.sql.DataSource"

这样一句。然后在去细看JndiObjectFactoryBean类中的afterPropertiesSet方法的具体代码所以一切都明了了。

 

综上所述，这里主要还是要对Spring的FactoryBean模式的理解最为重要。