当 netbeans 项目转换为可执行 jar 时，JLabel.setVisible 不起作用

Question

我有一个带有“显示”按钮的简单表单，单击时会读取“content.txt”文件的内容，并在标签中显示文本，该标签隐藏并在单击按钮时可见。

该项目在作为 netbeans 项目运行时有效。通过 netbeans IDE，我构建项目并将其打包到一个 jar 中。当作为 jar 执行时，无法看到此基本功能。

这是完整的代码，“content.txt”位于 src 文件夹中。

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.net.URISyntaxException;
import java.net.URL;

public class TestForm extends javax.swing.JFrame {

    public TestForm() {
        initComponents();
        jLabel1.setVisible(false);
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jButton1 = new javax.swing.JButton();
        jLabel1 = new javax.swing.JLabel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jButton1.setText("Display");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });

        jLabel1.setBorder(new javax.swing.border.MatteBorder(null));

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addGap(155, 155, 155)
                        .addComponent(jButton1))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(119, 119, 119)
                        .addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, 125, javax.swing.GroupLayout.PREFERRED_SIZE)))
                .addContainerGap(156, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(37, 37, 37)
                .addComponent(jButton1)
                .addGap(51, 51, 51)
                .addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, 96, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(89, Short.MAX_VALUE))
        );

        pack();
    }// </editor-fold>                        

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         

        try {
            URL dir_url = ClassLoader.getSystemResource("content.txt");
            File file = new File(dir_url.toURI());

            BufferedReader reader = new BufferedReader(new FileReader(file));
            String line = null;
            StringBuilder stringBuilder = new StringBuilder();
            String ls = System.getProperty("line.separator");

            while ((line = reader.readLine()) != null) {
                stringBuilder.append(line);
                stringBuilder.append(ls);
            }

            jLabel1.setText(stringBuilder.toString());

            jLabel1.setVisible(true);
        }
        catch(URISyntaxException ex){
            ex.printStackTrace();
        }
        catch(FileNotFoundException ex){
            ex.printStackTrace();
        }
        catch(IOException ex){
            ex.printStackTrace();
        }

    }                                        

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(TestForm.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(TestForm.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(TestForm.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(TestForm.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new TestForm().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JLabel jLabel1;
    // End of variables declaration                   
}

当我运行可执行 jar 时，它不会给我一个错误，但没有显示带有内容的标签。

当我使用命令行运行 jar 时，出现以下异常。

Exception in thread "main" java.lang.UnsupportedClassVersionError: TestForm : Unsupported major.minor version 52.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
    at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:482)

Answer 1

“java.lang.UnsupportedClassVersionError”是由于我的 netbeans 使用的是 Java 8 而我试图在运行 Java 7 的终端中运行它。

一旦我解决了上述问题并能够在终端中运行 jar 文件，我得到一个“java.lang.IllegalArgumentException：URI 不是分层的”，这是因为我试图访问资源“content.txt”这是在jar文件中。

我在读取文本文件内容的代码中进行了以下更改，并解决了问题。

InputStream stream = getClass().getResourceAsStream("content.txt");

BufferedReader reader = new BufferedReader(new InputStreamReader(stream));