Java 代码从不进入 try-catch 中的 try 块

Question

我目前正在创建一个项目来编写从音乐数据库中提供信息的客户端和服务。

我创建了一个部署良好的 MusicService.java 文件。 但是，我不确定代码是否正确。 然后我需要创建一个客户端。 当我运行客户端时，我不断收到从我的 MusicService.java 类中的 try catch 抛出的错误错误，说（“无法找到作曲家”）或（“无法找到光盘”）通过进行一些基本的调试，我知道尝试getByComposer 和 getByDisc 中的方法永远不会被输入并直接捕获。

我还知道在我的 client.java 方法中，由于我的打印语句在控制台中返回，因此正在输入 try。 然而，代码永远不会执行并直接进入 catch 异常，我不知道为什么。

我知道某个地方有错误，也许我没有打电话，但我花了几个小时来解决这个问题，似乎找不到问题所在。

我将把 2 种方法留在下面，如果有人可以提供帮助，将不胜感激。

MusicService.java 方法

package music;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class MusicService extends MusicServiceSkeleton {

 private final static String databaseHost = "mysql0.cs.stir.ac.uk";
 private final static String databaseName = "CSCU9YW";
 private final static String databasePassword = "rtk1";
 private final static String databaseUser = "rtk1";
 private final static String discTable = "music";


//define methods to implement the WSDL opeartions getByComposer() and getByDisc()

 public music.MultipleTracks getByComposer(music.Composer composer)throws ErrorFault
 {
     try {
         TrackDetail[]info=getByField("composer",composer.getComposer());
         MultipleTracks multiTrack = new MultipleTracks(); 
         TrackDetails multiRetrieve = new TrackDetails(); 
         multiRetrieve.setTracklist(info);
         multiTrack.setMultipleTracks(multiRetrieve);
         return multiTrack;
     }
     catch(Exception e)
     {
         throw(new ErrorFault("Unable to find composer"));
     }
 }

 public music.MultipleTracks getByDisc(music.Disc disc)throws ErrorFault
 {
     try {
         System.out.println("hello in getbbydisc");
         TrackDetail[]info=getByField("disc",disc.getDisc());
         MultipleTracks multiTrack = new MultipleTracks(); 
         TrackDetails multiRetrieve = new TrackDetails(); 
         multiRetrieve.setTracklist(info);
         multiTrack.setMultipleTracks(multiRetrieve);
         return multiTrack;
     }
     catch(Exception e)
     {
         throw(new ErrorFault("Unable to find disc"));
     }
 }



    private TrackDetail[] getByField(String field, String value) throws ErrorFault {
        try {
            if (value.length() == 0)
                throw (new Exception(field + " is empty"));
            Class.forName("com.mysql.jdbc.Driver").newInstance();
            String databaseDesignation = "jdbc:mysql://" + databaseHost + "/" + databaseName + "?user=" + databaseUser
                    + "&password=" + databasePassword;
            Connection connection = DriverManager.getConnection(databaseDesignation);
            Statement statement = connection.createStatement();
            String query = "SELECT disc, track, composer, work, title " + "FROM " + discTable + " " + "WHERE " + field
                    + " LIKE '%" + value + "%'";
            ResultSet result = statement.executeQuery(query);
            result.last();
            int resultCount = result.getRow();
            if (resultCount == 0)
                throw (new Exception(field + " '" + value + "' not found"));

            TrackDetail[] trackDetails = new TrackDetail[resultCount];
            result.beforeFirst();
            int resultIndex = 0;
            while (result.next()) {
                TrackDetail receiver = detailsReceived(result);
                trackDetails[resultIndex++]= receiver;
            }
            connection.close();
            return (trackDetails);
        } catch (Exception exception) {
            String errorMessage = "database access error - " + exception.getMessage();
            throw (new ErrorFault(errorMessage, exception));
        }
    }
    private TrackDetail detailsReceived(ResultSet current)throws SQLException
    {
    TrackDetail details = new TrackDetail();
    details.setDiscNumber(current.getString(1));
    details.setTrackNumber(current.getString(2));
    details.setComposerName(current.getString(3));
    details.setWorkName(current.getString(4));
    details.setTitleName(current.getString(5));

    return details;
    }

}

Client.java 方法

package music;

import music.MusicServiceStub.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Client extends JFrame implements ActionListener {
    private final static int contentInset = 5;
    private final static int trackColumns = 130;
    private final static int trackRows = 20;
    private final static int gridLeft = GridBagConstraints.WEST;
    private final static String programTitle = "Music Album";

    private GridBagConstraints contentConstraints = new GridBagConstraints();
    private GridBagLayout contentLayout = new GridBagLayout();
    private Container contentPane = getContentPane();
    private JButton discButton = new JButton("Check");
    private JLabel discLabel = new JLabel("Disc Number:");
    private JTextField discText = new JTextField(5);
    private JButton nameButton = new JButton("Check");
    private JLabel nameLabel = new JLabel("Composer/Artiste Name:");
    private JTextField nameText = new JTextField(16);
    private Font trackFont = new Font(Font.MONOSPACED, Font.PLAIN, 12);
    private JLabel trackLabel = new JLabel("Tracks:");
    private JTextArea trackArea = new JTextArea(trackRows, trackColumns);
    private JScrollPane trackScroller = new JScrollPane(trackArea);
    private MultipleTracks tracks;


// define here private variable for your Client Stub


    public Client() throws Exception  {
        contentPane.setLayout(contentLayout);
        addComponent(0, 0, gridLeft, nameLabel);
        addComponent(1, 0, gridLeft, nameText);
        addComponent(2, 0, gridLeft, nameButton);
        addComponent(0, 1, gridLeft, discLabel);
        addComponent(1, 1, gridLeft, discText);
        addComponent(2, 1, gridLeft, discButton);
        addComponent(0, 2, gridLeft, trackLabel);
        addComponent(0, 3, gridLeft, trackScroller);
        nameButton.addActionListener(this);
        discButton.addActionListener(this);
        trackArea.setFont(trackFont);
        trackArea.setEditable(false);

// instantiate your Client Stub and assign to private class variable declared above

}

    public static void main(String[] args) throws Exception {
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        int screenWidth = screenSize.width;
        int screenHeight = screenSize.height;

        Client window = new Client();
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setTitle(programTitle);
        window.pack();
        int windowWidth = window.getWidth();
        int windowHeight = window.getHeight();
        int windowX = (screenWidth - windowWidth) / 2;
        int windowY = (screenHeight - windowHeight) / 2;
        window.setLocation(windowX, windowY);
        window.setVisible(true);
    }

    private void addComponent(int x, int y, int position, JComponent component) {
        Insets contentInsets = new Insets(contentInset, contentInset, contentInset, contentInset);
        contentConstraints.gridx = x;
        contentConstraints.gridy = y;
        contentConstraints.anchor = position;
        contentConstraints.insets = contentInsets;
        if (component == trackArea || component == trackLabel)
            contentConstraints.gridwidth = GridBagConstraints.REMAINDER;
        contentLayout.setConstraints(component, contentConstraints);
        contentPane.add(component);
    }

    public void actionPerformed(ActionEvent event) {
        String trackRows = "";
        TrackDetail[] tracks;
        try {
            //System.out.println("Hello");
            if (event.getSource() == nameButton)
                tracks = getField("composer", nameText.getText());
            else if (event.getSource() == discButton)
                tracks = getField("disc", discText.getText());
            else
                return;
            trackRows += String.format("%4s %5s %-32s %-40s %-40s\n", "Disc", "Track", "Composer/Artist", "Work", "Title");
            for (int i = 0; i < tracks.length; i++) {
                TrackDetail trackDetail = tracks[i];
                trackRows += extractMusicData(trackDetail);


// extract the data for a track and append to trackRows variable


            }
        } catch (Exception exception) {
            String error = exception.getMessage();
            if (error == null) error = exception.toString();
            error = "could not get track - " + error;
            trackRows += error;
        }
        trackArea.setText(trackRows);
    }

    private String extractMusicData(TrackDetail track )
    {
        String albumName= track.getWorkName();
        String composer = track.getComposerName();
        String title= track.getTitleName();
        String trackNo= track.getTrackNumber().toString();
        String discNo= track.getDiscNumber().toString();

        String extractMusicTracks= String.format("%4s %5s %-32s %-40s %-40s\n", discNo, trackNo, composer, albumName, title);
        return extractMusicTracks; 
    } 

    private TrackDetail[] getField(String field, String value) throws Exception {

// define behaviour for method getField() to call the web service methods and receive the results
    try 
    {
        TrackDetail[] multiTrack; 
        MusicServiceStub stub=new MusicServiceStub();

        if(field.equals("disc"))
        {
            MusicServiceStub.Disc requestDisc = new MusicServiceStub.Disc(); 
            requestDisc.setDisc(value);
            MusicServiceStub.MultipleTracks message= stub.getByDisc(requestDisc); 
            multiTrack= extractedTracks(message);
            return multiTrack; 
        } else if(field.equals("composer"));
        {
            MusicServiceStub.Composer requestComposer = new MusicServiceStub.Composer(); 
            requestComposer.setComposer(value);
            MusicServiceStub.MultipleTracks message= stub.getByComposer(requestComposer); 
            multiTrack= extractedTracks(message);
            return multiTrack; 
        }
    } catch(Exception e)
    {
        System.out.println(e);
    }
return null; 
    }


    private TrackDetail[] extractedTracks(MultipleTracks tracks)
    {
        TrackDetails track= tracks.getMultipleTracks();
        TrackDetail[] multiTrack= track.getTracklist(); 
        return multiTrack;
    }
    }

Answer 1

您的错误处理方式已经结束：

1. 惯用的 java 规定异常以“异常”一词结尾。 不要使用名称“ErrorFault”。 除了不惯用语之外，这是一个同义反复。

2. 重新抛出时，传递原因，丢失括号： throw new ErrorFault("Unable to find composer", e); 除了不要将其命名为 ErrorFault。 现在您可以检查是什么导致 java 跳转到 catch 块。

请注意，java 不会无缘无故地随机跳转到捕获块。 该 try 块中的某些内容正在抛出，并且您通过抛出其他内容而不将原始内容作为原因传递来隐藏原因。

3. 永远不要捕获异常而只执行“System.out.println(e)” - 之后代码将继续，即使进程现在显然处于无效状态。 如果您不知道如何处理异常，最好的办法是在您的方法中添加一个“throws”子句以继续抛出它。 如果这不是一个选项，并且您也不想将其包装在适当的异常中，那么绝对的回退是throw new RuntimeException(e) ，而不是e.printStackTrace() ，或者更糟糕的是，信息消除System.out.println(e); - 更新您的 IDE 模板！

一旦您应用了上述建议，问题就应该清楚了。 如果不是，至少现在你会得到一个包含更多细节的堆栈跟踪； 使用该信息将其作为问题发布在这里（或更新您的问题），也许有人可以帮助您弄清楚发生了什么。