BorderLayout中FlowLayout换行覆盖JTextArea，如何实现JTextArea自适应？

问题原因 #

默认的FlowLayout在组件换行后，容器的首选高度不会自动更新，导致BorderLayout无法感知顶部面板需要更多垂直空间，进而让顶部面板溢出并覆盖下方的JTextArea区域。

解决方案 #

推荐使用改进版的WrapLayout（FlowLayout子类），它能正确计算换行后的容器首选高度，让BorderLayout自动调整各区域的大小。

第一步：实现WrapLayout

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

public class WrapLayout extends FlowLayout {
    public WrapLayout() {
        super();
    }

    public WrapLayout(int align) {
        super(align);
    }

    public WrapLayout(int align, int hgap, int vgap) {
        super(align, hgap, vgap);
    }

    @Override
    public Dimension preferredLayoutSize(Container target) {
        return layoutSize(target, true);
    }

    @Override
    public Dimension minimumLayoutSize(Container target) {
        Dimension dim = layoutSize(target, false);
        dim.width -= (getHgap() + 1);
        return dim;
    }

    private Dimension layoutSize(Container target, boolean preferred) {
        synchronized (target.getTreeLock()) {
            int targetWidth = target.getSize().width;
            if (targetWidth == 0) {
                targetWidth = Integer.MAX_VALUE;
            }

            int hgap = getHgap();
            int vgap = getVgap();
            Insets insets = target.getInsets();
            int horizontalInsetsAndGap = insets.left + insets.right + (hgap * 2);
            int maxWidth = targetWidth - horizontalInsetsAndGap;

            Dimension dim = new Dimension(0, 0);
            int rowWidth = 0;
            int rowHeight = 0;

            int nmembers = target.getComponentCount();

            for (int i = 0; i < nmembers; i++) {
                Component m = target.getComponent(i);
                if (m.isVisible()) {
                    Dimension d = preferred ? m.getPreferredSize() : m.getMinimumSize();
                    if (rowWidth + d.width > maxWidth) {
                        addRow(dim, rowWidth, rowHeight);
                        rowWidth = 0;
                        rowHeight = 0;
                    }
                    if (rowWidth != 0) {
                        rowWidth += hgap;
                    }
                    rowWidth += d.width;
                    rowHeight = Math.max(rowHeight, d.height);
                }
            }

            addRow(dim, rowWidth, rowHeight);

            dim.width += horizontalInsetsAndGap;
            dim.height += insets.top + insets.bottom + vgap * 2;

            Container scrollPane = SwingUtilities.getAncestorOfClass(JScrollPane.class, target);
            if (scrollPane != null) {
                dim.width -= (hgap + 1);
            }

            return dim;
        }
    }

    private void addRow(Dimension dim, int rowWidth, int rowHeight) {
        dim.width = Math.max(dim.width, rowWidth);
        if (dim.height > 0) {
            dim.height += getVgap();
        }
        dim.height += rowHeight;
    }
}

第二步：修改原代码的布局

把顶部JPanel的FlowLayout替换成WrapLayout：

// 原代码
// JPanel jPanel = new JPanel(new FlowLayout());
// 修改为：
JPanel jPanel = new JPanel(new WrapLayout());

效果 #

窗口缩小时，顶部面板换行后会自动扩展高度，BorderLayout会自动调整CENTER区域的JTextArea大小，避免被覆盖。

内容的提问来源于stack exchange，提问作者nobody