为什么我会收到 com.vaadin.event.ListenerMethod$MethodException？

Question

我的小型数据库（DERBY）应用程序有很大问题。当我单击按钮时，我收到这样的错误：

2011-12-25 04:21:38 com.vaadin.Application terminalError
SEVERE: Terminal error:
com.vaadin.event.ListenerMethod$MethodException
Cause: java.lang.NullPointerException
    at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:532)
    at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:164)
    at com.vaadin.ui.AbstractComponent.fireEvent(AbstractComponent.java:1219)
    at com.vaadin.ui.Button.fireClick(Button.java:550)
    at com.vaadin.ui.Button.changeVariables(Button.java:217)
    at com.vaadin.terminal.gwt.server.AbstractCommunicationManager.changeVariables(AbstractCommunicationManager.java:1445)
    at com.vaadin.terminal.gwt.server.AbstractCommunicationManager.handleVariableBurst(AbstractCommunicationManager.java:1393)
    at com.vaadin.terminal.gwt.server.AbstractCommunicationManager.handleVariables(AbstractCommunicationManager.java:1312)
    at com.vaadin.terminal.gwt.server.AbstractCommunicationManager.doHandleUidlRequest(AbstractCommunicationManager.java:763)
    at com.vaadin.terminal.gwt.server.CommunicationManager.handleUidlRequest(CommunicationManager.java:296)
    at com.vaadin.terminal.gwt.server.AbstractApplicationServlet.service(AbstractApplicationServlet.java:501)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:395)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:250)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
    at com.example.expert_system.Knowledge.insertPhone(Knowledge.java:280)
    at com.example.expert_system.Knowledge.buttonClick(Knowledge.java:262)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:512)
    ... 27 more

仅当我尝试从类 Database_engine 调用该方法时，我才会收到此错误。

Database_engine.java：

private static String dbURL = "jdbc:derby://localhost:1527/Knowledge;create=true;user=erni;password=2ngxc3";
private static String tableName = "ERNI.PHONE";
private static Connection conn = null;
private static Statement stmt = null;




private void createConnection()
{
    try
    {
        Class.forName("org.apache.derby.jdbc.ClientDriver").newInstance();
        //Get a connection
        conn = DriverManager.getConnection(dbURL); 
    }
    catch (Exception except)
    {
        except.printStackTrace();
    }
}


public void insertPhone(String producer, String model, String construction, String display_size, String system)
{

    createConnection();
    try
    {
        stmt = conn.createStatement();
        stmt.execute("insert into " + tableName + "(\"producer\",\"model\", \"construction\",\"dispaly_size\",\"system\") values (" + producer + "','" + model + "','" + construction + "','" + display_size + "','" + system +"')");
        stmt.close();
        //INSERT INTO ERNI.PHONE ("producer", "model", "construction", "display_size", "system") VALUES ('placek', 'na', 'oleju', 'mama ', 'krzyczy');

    }
    catch (SQLException sqlExcept)
    {
        sqlExcept.printStackTrace();
        //System.err.println("Exception: "+sqlExcept.getMessage());
    } finally {
        closeConnection();
    }

}



private void closeConnection()
{
    try
    {
        if (stmt != null)
        {
            stmt.close();
        }
        if (conn != null)
        {
            DriverManager.getConnection(dbURL + ";shutdown=true");
            conn.close();
        }           
    }
    catch (SQLException sqlExcept)
    {

    }

}

以及表单方法的代码：

public void buttonClick(ClickEvent event) {
        Button source = event.getButton();
      //  
       if (source == button_2) {


            insertPhone();
            getWindow().showNotification("Open " + producerBox.getValue().toString() +" "+ 
                     model.getValue().toString()+" "+ 
                     constructionBox.getValue().toString()+" "+ 
                     displayBox.getValue().toString()+" "+ 
                     systemBox.getValue().toString());

            setReadOnly(true);  
            getWindow().showNotification(systemBox.getInputPrompt());
        } else if (source == button_1) {
            discard();
            setReadOnly(true);
            getWindow().showNotification("clear executed");
        }
    }

    private void insertPhone() {
        Database_engine addPhone = new Database_engine();
        addPhone.insertPhone(producerBox.getValue().toString(), 
                             model.getValue().toString(), 
                             constructionBox.getValue().toString(), 
                             displayBox.getValue().toString(), 
                             systemBox.getValue().toString());

    }
    private void discard() {
        producerBox.setValue(null);
        model.setValue("");
        constructionBox.setValue(null);
        displayBox.setValue(null);
        systemBox.setValue(null);
        otherColSelect.setValue(null);
        comColSelect.setValue(null);

    }

请告诉我我做错了什么。

---

好的，我已经检查了你建议的所有内容。 当我调试代码时，我得到这样的答案“找不到源”，

我编写了一个主要方法来测试该类，我得到：

org.apache.derby.client.am.SqlException: Syntax error: Encountered "\',\'" at line 1, column 83.
    at org.apache.derby.client.am.Statement.completeSqlca(Unknown Source)
    at org.apache.derby.client.am.Statement.completeExecuteImmediate(Unknown Source)
    at org.apache.derby.client.net.NetStatementReply.parseEXCSQLIMMreply(Unknown Source)
    at org.apache.derby.client.net.NetStatementReply.readExecuteImmediate(Unknown Source)
    at org.apache.derby.client.net.StatementReply.readExecuteImmediate(Unknown Source)
    at org.apache.derby.client.net.NetStatement.readExecuteImmediate_(Unknown Source)
    at org.apache.derby.client.am.Statement.readExecuteImmediate(Unknown Source)
    at org.apache.derby.client.am.Statement.flowExecute(Unknown Source)
    at org.apache.derby.client.am.Statement.executeX(Unknown Source)
    at org.apache.derby.client.am.Statement.execute(Unknown Source)
    at com.example.expert_system.Database_engine.insertPhone(Database_engine.java:56)
    at com.example.expert_system.Database_engine.main(Database_engine.java:24)

第 56 行是 stmt.execute("insert into " + tableName + " (生产者，模型、构造、display_size、系统）值 (" + db_ Producer + "','" + db_model + "','" + db_construction + "','" + db_display_size + "','" + db_system +"')");

但一切正常。我想是这样吗？

有什么建议吗？

Answer 1

正如它所说，你的 SQL 有语法错误。在你的 SQL 中你有：

... 值 (" + db_ Producer + "','" ...

其中包含不匹配的单引号。

Answer 2

您的 createConnection 方法可能不会成功，或者您的 createStatement 调用不会成功，从而将“conn”保留为 null。