最近在看《Spring In Action》，在做測試時，碰到了一個問題，那就是：
同一個Class，卻被不同的Class Loader 加載，出現的異常爲：
java.lang.ClassCastException: spring.in.action.bean.chapter05.OrderDetail cannot be cast to spring.in.action.bean.chapter05.OrderDetail.
Spring的配置文件內容如下：
<?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-3.1.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.1.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.1.xsd">
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<list>
<!-- <value>classpath:/*/*/datasource.properties</value> -->
<value>F:\Eclipse2\spring-in-action\config\datasource.properties</value>
</list>
</property>
</bean>
<bean id="dataSource" class="org.apache.tomcat.dbcp.dbcp.BasicDataSource">
<property name="driverClassName" value="${datasource.driverClassName}"></property>
<property name="url" value="${datasource.url}"></property>
<property name="username" value="${datasource.username}"></property>
<property name="password" value="${datasource.password}"></property>
<property name="maxActive" value="5"></property>
<property name="minIdle" value="3"></property>
</bean>
<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
<property name="dataSource" ref="dataSource"></property>
<property name="jpaVendorAdapter">
<bean class="org.springframework.orm.jpa.vendor.EclipseLinkJpaVendorAdapter">
<property name="database" value="MYSQL"></property>
<property name="showSql" value="true"></property>
<property name="generateDdl" value="true"></property>
</bean>
</property>
<property name="jpaProperties">
<props>
<prop key="eclipselink.ddl-generation">NONE</prop>
</props>
</property>
<property name="loadTimeWeaver">
<bean class="org.springframework.instrument.classloading.SimpleLoadTimeWeaver"></bean>
</property>
</bean>
<bean id="orderDetailDao" class="spring.in.action.bean.chapter05.OrderDetailDaoImpl">
<property name="entityManagerFactory" ref="entityManagerFactory"></property>
</bean>
</beans>
然後，具體的將各個Class的ClassLoader打印出來看了下，發現真的不一樣：
List<OrderDetail> orderDetailList = getJpaTemplate().find("select orderDetail from OrderDetail orderDetail");
if(!orderDetailList.isEmpty()){
for(Object obj : orderDetailList){
System.out.println("Element:"+obj.getClass().getClassLoader());
System.out.println("order:"+OrderDetail.class.getClassLoader());
break;
}
}else{
System.out.println("orderDetail list is null");
}
結果是：
Element: org.springframework.instrument.classloading.SimpleInstrumentableClassLoader@15c07d8
order: sun.misc.Launcher$AppClassLoader@19821f從結果可以看出，一個OrderDetail用的是sun提供的ClassLoader，另一個用的是Spring的，而Spring這個明顯是從Sun的ClassLoader中繼承過來的。
所以解決方法是重寫 SimpleLoadTimeWeaver:
public class MyLoadTimeWeaver extends SimpleLoadTimeWeaver {
@Override
public ClassLoader getInstrumentableClassLoader() {
// TODO Auto-generated method stub
return super.getInstrumentableClassLoader().getParent();
// return super.getInstrumentableClassLoader();
}
}然後將配置文件中的LoadTimeWeaver屬性的值變成：
<property name="loadTimeWeaver">
<bean class="spring.in.action.bean.util.MyLoadTimeWeaver"></bean>
</property>這樣問題就解決了。
最後的Spring配置文件變爲：
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.1.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.1.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.1.xsd">
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<list>
<!-- <value>classpath:/*/*/datasource.properties</value> -->
<value>F:\Eclipse2\spring-in-action\config\datasource.properties</value>
</list>
</property>
</bean>
<bean id="dataSource" class="org.apache.tomcat.dbcp.dbcp.BasicDataSource">
<property name="driverClassName" value="${datasource.driverClassName}"></property>
<property name="url" value="${datasource.url}"></property>
<property name="username" value="${datasource.username}"></property>
<property name="password" value="${datasource.password}"></property>
<property name="maxActive" value="5"></property>
<property name="minIdle" value="3"></property>
</bean>
<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
<property name="dataSource" ref="dataSource"></property>
<property name="jpaVendorAdapter">
<bean class="org.springframework.orm.jpa.vendor.EclipseLinkJpaVendorAdapter">
<property name="database" value="MYSQL"></property>
<property name="showSql" value="true"></property>
<property name="generateDdl" value="true"></property>
</bean>
</property>
<property name="jpaProperties">
<props>
<prop key="eclipselink.ddl-generation">NONE</prop>
</props>
</property>
<property name="loadTimeWeaver">
<bean class="spring.in.action.bean.util.MyLoadTimeWeaver"></bean>
</property>
</bean>
<bean id="orderDetailDao" class="spring.in.action.bean.chapter05.OrderDetailDaoImpl">
<property name="entityManagerFactory" ref="entityManagerFactory"></property>
</bean>
</beans>===========================================================================================================
那可不可以不用SimpleLoadTimeWeaver呢？因爲用了它，我們就會改變Class的ClassLoader，造成同一個Class會被不同的ClassLoader加載。試了下在EntityManagerFactory的bean中，去掉loadtimeWeaver屬性，得到的卻是另一個異常：
java.lang.IllegalStateException: Cannot apply class transformer without LoadTimeWeaver specified這個問題沒有去研究，希望有研究的朋友，可以分享下