如何使用@Resource注释在Java文件中注入数据源？但是在Servlet中可以正常工作

Question

我正在尝试使用@Resource注释在Java文件中注入数据源，它给了我一个NullPointerException。令我困惑的是，当我把这个@Resource放在一个Servlet中时，它运行正常，我从数据库中获取数据。作为重构工作的一部分，我只是试图将这个@Resource注释从我的Servlet控制器调用到服务类，它给了我NPE。

ProductPageController.java

@WebServlet(name = "ProductPageController", urlPatterns = {"/" + AppConstants.PRODUCT_PAGE_CONTROLLER})
public class ProductPageController extends HttpServlet {

    private static final Logger LOG = Logger.getLogger(ProductPageController.class);

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        processRequest(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        processRequest(request, response);
    }

    protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        LOG.debug("Into the ProductPageController...");

        //fetch the product from the db based on productId parameter
        String productId = GenericUtils.getParameter(request, "productId");
        request.setAttribute("product", new ProductService().getProduct(new Integer(productId)));

        //show the page now
        showPage(request, response, PRODUCT_PAGE_URL);
    }

    private void showPage(HttpServletRequest request, HttpServletResponse response, String viewName) throws ServletException, IOException {
        LOG.debug("Displaying " + viewName + " page now...");
        request.getServletContext().getRequestDispatcher(viewName).forward(request, response);
    }
}

ProductService.java

public class ProductService {

    private static final Logger LOG = Logger.getLogger(ProductService.class);

    @Resource(name = "jdbc/istore-db")
    private DataSource dataSource;

    public Product getProduct(int productId) {
        Product product = null;
        try {
            product = new MasterDao(dataSource).getProduct(productId);
        } catch (SQLException sqle) {
            LOG.error("Exception while fetching product based on productId: " + productId);
            LOG.error(sqle);
        }
        LOG.debug("product = " + product);
        return product;
    }
}

错误日志：

 java.lang.NullPointerException
    at com.istore.dao.MasterDao.getProduct(MasterDao.java:75)
    at com.istore.service.ProductService.getProduct(ProductService.java:38)
    at com.istore.web.controllers.ProductPageController.processRequest(ProductPageController.java:35)
    at com.istore.web.controllers.ProductPageController.doGet(ProductPageController.java:22)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:618)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:537)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1085)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:658)
    at org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1556)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1513)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)

Answer 1

如果您使用new ProductService()自己实例化该类，则不会发生依赖注入。

您必须在servlet中注入ProductService。

@Inject
private ProductService productService

Answer 2

直接使用JNDI API可以绕过@Resource注解资源注入不工作的问题

try {
   Context initialCtxt = new InitialContext();
   dataSource = (DataSource) initialCtxt.lookup("java:comp/env/jdbc/istore-db");
} catch (NamingException ex) {
   LOG.error("Exception while fetching the data source: jdbc/istore-db");
   LOG.error(ex);
}

并且您可以将此代码放在您的 ProductService 构造函数中