JavaSwing透明JBAND问题

我有一个jlayeredpane,其中添加了3个jpanel。

我使jpanels透明(没有设置背景和setopaque(false))。我在JPanel上画线,只有最后一个JPanel上添加的线是可见的。其他jpanel的行在顶部jpanel中是不可见的(即使我在添加它们时添加了不同的zindex)。

有谁知道解决办法吗?为什么它们不透明?

我已经创建了一个小测试程序(3个类)。 (testjpanel和testjpanel1画了一条线,但位置不同,但我只看到最后添加的jpanel的线。我看不到两行,因为它不透明:()

JAVA

import java.awt.BorderLayout;
import java.awt.Color;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLayeredPane;

public class Main extends JFrame {
  public Main() {
    setSize(400, 350);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    JLayeredPane lp = getLayeredPane();
    lp.setLayout(new BorderLayout());
    TestJPanel top = new TestJPanel();
    top.setOpaque(false);
    TestJPanel middle = new TestJPanel();
    middle.setOpaque(false);
    TestJPanel1 bottom = new TestJPanel1();
    bottom.setOpaque(false);

    lp.add(middle, BorderLayout.CENTER, new Integer(4));
    lp.add(top, BorderLayout.CENTER, new Integer(3));
    lp.add(bottom, BorderLayout.CENTER, new Integer(2));
    // the last one I have added (bottom) is visible and can't see the others through it

    setVisible(true);
  }

  public static void main(String[] args) {
    new Main();

  }
}

testjpanel.java测试面板

import java.awt.Graphics;


public class TestJPanel extends javax.swing.JPanel {

    /** Creates new form TestJPanel */
    public TestJPanel() {
        initComponents();
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);

        g.drawLine(25, 0, 25, 50);
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 400, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 300, Short.MAX_VALUE)
        );
    }// </editor-fold>


    // Variables declaration - do not modify
    // End of variables declaration

}

testjpanel1.java测试

import java.awt.Graphics;


public class TestJPanel1 extends javax.swing.JPanel {

    /** Creates new form TestJPanel */
    public TestJPanel1() {
        initComponents();
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);

        g.drawLine(50, 0, 50, 50);
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 400, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 300, Short.MAX_VALUE)
        );
    }// </editor-fold>


    // Variables declaration - do not modify
    // End of variables declaration

}

我真的希望有人能帮我解决这个问题。