在spring中使用jdbcTemplate时获取nullpointerException

Question

I am new to Spring, and working on project where I need to get some data from a Database. I am using tomcat server and using JNDI DB connection pooling. Below is my code and Spring configuration. I am getting a NullPointerException because jdbcTemplate is null .

public class AppConfig 
{
@Autowired
private JdbcTemplate jdbcTemplate;
private static AppConfig config=null;
private HashMap<String,  String> dbAppParameterValuesCacheMap;
public AppConfig()
{
    cacheConfig();

}

public boolean cacheConfig()
{
    dbAppParameterValuesCacheMap = null;
    List<Map<String, Object>> appConfigMapList=null;
    String parameterType="PP_APP_CONFIG";
    try
    {
        appConfigMapList= jdbcTemplate
            .queryForList("SELECT  Parameter_Value, Parameter_Name FROM PP_Application_Parameter where PARAMETER_TYPE='"+parameterType+"'");
    }
    catch(NullPointerException ex)
    {
        System.out.println("here");
        ex.printStackTrace();
    }
    if (dbAppParameterValuesCacheMap == null)
        dbAppParameterValuesCacheMap = new HashMap<String,String>();
    for(Map<String, Object> configMap:appConfigMapList)
    {
            dbAppParameterValuesCacheMap.put((String)configMap.get("Parameter_Name"), (String)configMap.get("Parameter_Value"));
    }

    return true;
}
}

My Spring configuration file contains:

<bean id="dbDataSource" class="org.springframework.jndi.JndiObjectFactoryBean">
    <property name="jndiName" value="java:comp/env/jdbc/PP_DATASOURCE" />
</bean>
<bean id="jdbcTemplate" class="org.springframework.jdbc.core.JdbcTemplate" lazy-init="false">
    <property name="dataSource" ref="dbDataSource"></property>
</bean>
<bean id="config" class="AppConfig" scope="singleton">
</bean>

JNDI has been created successfully. I am getting a NullPointerException when it tries to execute the line

appConfigMapList= jdbcTemplate
            .queryForList("SELECT  Parameter_Value, Parameter_Name FROM PP_Application_Parameter where PARAMETER_TYPE='"+parameterType+"'");

Answer 1

As per the documentation of Autowired , injection happens after construction of the bean.

Fields are injected right after construction of a bean, before any config methods are invoked. Such a config field does not have to be public.

Since your code is attempting to reference jdbcTemplate from the constructor, it has not yet been injected, and therefore it is null .

If your goal is to run some extra initialization code after the auto-wired dependencies are guaranteed to be in place, then one way to do that is to annotate a different method from the constructor with PostContruct .

Answer 2

Consider this change:

public AppConfig()
{
}

@PostConstruct
public boolean cacheConfig()
{

This will move access to jdbcTemplate to a time after the autowirings has been done by Spring while keeping your semantics (run cacheConfig right after object construction) in place.

@Autowired fields are null while the constructor runs.