Question

我是EJB新手，因此我正在执行示例EJB项目，如教程Java EE 6 tutorial

以下是豆子的外观：

Converter.java

package converter.ejb;

import java.math.BigDecimal;
import javax.ejb.Remote;

@Remote
public interface Converter {
    public BigDecimal dollarToYen(BigDecimal dollars);
    public BigDecimal yenToEuro(BigDecimal yen);
}

ConverterBean.java

package converter.ejb;

import java.math.BigDecimal;
import javax.ejb.LocalBean;
import javax.ejb.Stateless;

@Stateless
public class ConverterBean {

    private BigDecimal yenRate = new BigDecimal("115.3100");
    private BigDecimal euroRate = new BigDecimal("0.0071");

    public BigDecimal dollarToYen(BigDecimal dollars) {
        BigDecimal result = dollars.multiply(yenRate);
        return result.setScale(2, BigDecimal.ROUND_UP);
    }

    public BigDecimal yenToEuro(BigDecimal yen) {
        BigDecimal result = yen.multiply(euroRate);
        return result.setScale(2, BigDecimal.ROUND_UP);
    }
}

ConverterClient.java

package converter.ejb;

import java.math.BigDecimal;
import javax.ejb.EJB;

public class ConverterClient {

    @EJB
    private static Converter converter;

    public ConverterClient(String[] args) {
    }

    public static void main(String[] args) {
        ConverterClient client = new ConverterClient(args);
        client.doConversion();
    }

    public void doConversion() {
        try {
            BigDecimal param = new BigDecimal("100.00");
            BigDecimal yenAmount = converter.dollarToYen(param);

            System.out.println("$" + param + " is " + yenAmount
                    + " Yen.");
            BigDecimal euroAmount = converter.yenToEuro(yenAmount);
            System.out.println(yenAmount + " Yen is " + euroAmount
                    + " Euro.");

            System.exit(0);
        } catch (Exception ex) {
            System.err.println("Caught an unexpected exception!");
            ex.printStackTrace();
        }
    }
}

我有一个Web客户端，它是一个JSP页面： converterweb.jsp

<%@ page import="converter.ejb.Converter,
            java.math.*, javax.naming.*"%>    
 <%!
    private Converter converter = null;
    public void jspInit() {
        try {
            InitialContext ic = new InitialContext();
            converter = (Converter)
                    ic.lookup(Converter.class.getName());
        } catch (Exception ex) {
            System.out.println("Couldn’t create converter bean."+
                    ex.getMessage());
        }
    }
        public void jspDestroy() {
        converter = null;
    }
%>
<html>
    <head>
        <title>Converter</title>
    </head>

    <body bgcolor="white">
        <h1>Converter</h1>
        <hr>
        <p>Enter an amount to convert:</p>
        <form method="get">
            <input type="text" name="amount" size="25">
            <br>
            <p>
            <input type="submit" value="Submit">
            <input type="reset" value="Reset">
        </form>

        <%
            String amount = request.getParameter("amount");
            if ( amount != null && amount.length() > 0 ) {
                BigDecimal d = new BigDecimal(amount);

                BigDecimal yenAmount = converter.dollarToYen(d);
        %>
        <p>
        <%= amount %> dollars are  <%= yenAmount %>  Yen.
        <p>
        <%
                BigDecimal euroAmount =
                        converter.yenToEuro(yenAmount);
        %>
        <%= amount %> Yen are <%= euroAmount %>  Euro.
        <%
            }
        %>
    </body>
</html>

但是每当我尝试执行这个项目时，我都会得到一个NullPointerException。我使用Websphere应用服务器。有人可以说明为什么会发生这种情况以及如何纠正它？

编辑：错误日志

[12/10/12 14:21:26:956 IST] 0000001f webapp        E com.ibm.ws.webcontainer.webapp.WebApp logServletError SRVE0293E: [Servlet Error]-[/converterweb.jsp]: java.lang.NullPointerException
    at com.ibm._jsp._converterweb._jspService(_converterweb.java:118)
    at com.ibm.ws.jsp.runtime.HttpJspBase.service(HttpJspBase.java:99)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:668)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:1188)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:763)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:454)
    at com.ibm.ws.webcontainer.servlet.ServletWrapperImpl.handleRequest(ServletWrapperImpl.java:178)
    at com.ibm.wsspi.webcontainer.servlet.GenericServletWrapper.handleRequest(GenericServletWrapper.java:122)
    at com.ibm.ws.jsp.webcontainerext.AbstractJSPExtensionServletWrapper.handleRequest(AbstractJSPExtensionServletWrapper.java:205)
    at com.ibm.ws.webcontainer.filter.WebAppFilterManager.invokeFilters(WebAppFilterManager.java:1020)
    at com.ibm.ws.webcontainer.servlet.CacheServletWrapper.handleRequest(CacheServletWrapper.java:87)
    at com.ibm.ws.webcontainer.WebContainer.handleRequest(WebContainer.java:895)
    at com.ibm.ws.webcontainer.WSWebContainer.handleRequest(WSWebContainer.java:1662)
    at com.ibm.ws.webcontainer.channel.WCChannelLink.ready(WCChannelLink.java:195)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleDiscrimination(HttpInboundLink.java:452)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleNewRequest(HttpInboundLink.java:511)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.processRequest(HttpInboundLink.java:305)
    at com.ibm.ws.http.channel.inbound.impl.HttpICLReadCallback.complete(HttpICLReadCallback.java:83)
    at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:165)
    at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:217)
    at com.ibm.io.async.AsyncChannelFuture.fireCompletionActions(AsyncChannelFuture.java:161)
    at com.ibm.io.async.AsyncFuture.completed(AsyncFuture.java:138)
    at com.ibm.io.async.ResultHandler.complete(ResultHandler.java:204)
    at com.ibm.io.async.ResultHandler.runEventProcessingLoop(ResultHandler.java:775)
    at com.ibm.io.async.ResultHandler$2.run(ResultHandler.java:905)
    at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1659)

Answer 1

您的代码充满了错误：

EJB依赖注入适用于由容器管理的组件。您的ConverterClient上没有注释（我假设您没有在xml描述符中执行任何其他EJB配置）。所以容器不知道它，不能注射。
您的转换器声明为静态。这不起作用。
您正在尝试通过Remote接口访问声明为@LocalBean的bean（这意味着它公开了无接口视图）。这也行不通。尝试直接使用ConverterBean或避免使用@LocalBean并使用@Remote（Converter.class）。

也许我错过了什么。希望这有点帮助。

错误500：运行EJB示例项目时出现java.lang.NullPointerException

1 个答案: