如何使BoxLayout正确左对齐？

Question

我创建了一个Box，其中包含JLabel，以及一个JScrollPane和JTextArea。但是，JLabel的左侧总是有一些空间：

完整的演示代码：

import java.awt.*;
import javax.swing.*;

public class BoxAlignmentTest extends JFrame {

    public static void main(String[] args) {
        BoxAlignmentTest test = new BoxAlignmentTest();
        test.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        test.setSize(500, 200);
        test.setVisible(true);
    }

    public BoxAlignmentTest() throws HeadlessException {
        Box box = Box.createVerticalBox();
        setContentPane(box);

        JLabel label = new JLabel("This label isn't fully left-aligned.");
        label.setOpaque(true);
        label.setBackground(Color.orange);
        label.setAlignmentX(Component.LEFT_ALIGNMENT);  // Set left alignment

        box.add(label);
        box.add(new JScrollPane(new JTextArea("This is a text area.")));
    }
}

Answer 1

使用setBorder（BorderFactory.createEmptyBorder（int顶部，int左侧，int底部，int右侧）;参考：https://docs.oracle.com/javase/7/docs/api/javax/swing/BorderFactory.html

Answer 2

How to Use BoxLayout (The Java™ Tutorials > Creating a GUI With JFC/Swing > Laying Out Components Within a Container)X对齐不仅影响组件之间的相对位置，还影响组件（作为一个组）在其容器中的位置。

因此，不仅对于setAlignmentX(Component.LEFT_ALIGNMENT)，而且对于JLabel，都需要JScrollPane。

import java.awt.*;
import javax.swing.*;

public class BoxAlignmentTest2 extends JFrame {
  public static void main(String[] args) {
    EventQueue.invokeLater(() -> {
      BoxAlignmentTest2 test = new BoxAlignmentTest2();
      test.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
      test.setSize(500, 200);
      test.setVisible(true);
    });
  }

  public BoxAlignmentTest2() throws HeadlessException {
    JLabel label = new JLabel("This label isn't fully left-aligned.");
    label.setOpaque(true);
    label.setBackground(Color.orange);
    label.setAlignmentX(Component.LEFT_ALIGNMENT); // Set left alignment

    JScrollPane scroll = new JScrollPane(new JTextArea("This is a text area."));
    scroll.setAlignmentX(Component.LEFT_ALIGNMENT); // <- add

    Box box = Box.createVerticalBox();
    box.add(label);
    box.add(scroll);

    add(box); // = getContentPane().add(box, BorderLayout.CENTER);
  }
}