JAVA：无法在过滤器中获取上下文参数

Question

我在 web.xml 中有一个过滤器和参数

web.xml 是这样的：

    <filter>
    <description>
    </description>
    <display-name>AllClassFilter</display-name>
    <filter-name>AllClassFilter</filter-name>
    <filter-class>com.datval.homework.AllClassFilter</filter-class>
    <init-param>
        <param-name>DB_URL</param-name>
        <param-value>jdbc:derby:C:/Users/admin/workspace/homework03/homework/databases/StudentsDB;create=true</param-value>
    </init-param>
    <init-param>
        <param-name>DB_DIALECT</param-name>
        <param-value>org.hibernate.dialect.DerbyDialect</param-value>
    </init-param>
    <init-param>
        <param-name>DB_DRIVER</param-name>
        <param-value>org.apache.derby.jdbc.EmbeddedDriver</param-value>
    </init-param>
</filter>

映射运行良好。 但我无法在过滤器中获取此参数。

    public void init(FilterConfig config) throws ServletException {
    // TODO Auto-generated method stub
    debugMessage = config.getInitParameter("debugMessage");
    ctx = config.getServletContext();
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
    // TODO Auto-generated method stub
    // place your code here

    ctx.log("Start - " + debugMessage);
    String myDbUrl =  ctx.getInitParameter("DB_URL");
    String DB_DIALECT = ctx.getInitParameter("DB_DIALECT");
    String DB_DRIVER = ctx.getInitParameter("DB_DRIVER");

    Map<String,String> pr = new HashMap<String,String>();
    pr.put("hibernate.connection.url", myDbUrl);
    pr.put("hibernate.dialect", DB_DIALECT);
    pr.put("hibernate.connection.driver_class", DB_DRIVER);

    EntityManagerFactory emf = Persistence.createEntityManagerFactory("students",pr);
    EntityManager em = emf.createEntityManager();
    request.setAttribute("em", em);
    chain.doFilter(request, response);
    em.close();
    ctx.log("end - " + debugMessage);
}

我检查过，myDbUrl 为空。我做错了什么？任何想法？ 对不起代码，我稍后会改变它:)

Answer 1

DB_URL 是FilterConfig 中的参数，而不是ServletContext。从传递给init() 方法的FilterConfig 实例访问它。