為什么 Java Swing 組件顯示為非輕量級，即使它們應該是輕量級的？

Question

我很好奇... 為什么JComponent.isLightweightComponent(Component c) 方法在傳遞JLabel、JButton 等swing 組件時會返回false？ 根據我閱讀的所有內容，這些組件應該是輕量級的。

這是否表明他們實際上是重量級的（絕對不應該是）？

或者 isLightweightComponent() 方法是否以某種方式被破壞？

還是我不了解 isLightweightComponent() 方法？

試試下面的代碼，看看我的意思...

import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class SwingLightweightTest {

    public static void main(String[] args) {
        // why do swing components show that they not Lightweight?
        // does this mean that they are Heavywight?
        // or is the .isLightweightComponent() method broken?

        JLabel jLabel = new JLabel();
        testLightweight(jLabel);

        JButton jButton = new JButton();
        testLightweight(jButton);

        JPanel jPanel = new JPanel();
        testLightweight(jPanel);
    }

    private static void testLightweight(JComponent comp) {
        String isIsnot;

        isIsnot = JComponent.isLightweightComponent(comp) ? "IS ": "IS NOT ";
        System.out.println(comp.getUIClassID() + " \t" + isIsnot + "a lightweight component");      

    }
}

它返回以下內容：

LabelUI 不是輕量級組件

ButtonUI 不是輕量級組件

PanelUI 不是輕量級組件

Answer 1

isLightweightComponent方法實際上不是檢查組件本身是否是輕量級的，而是檢查組件的“對等體”是否是輕量級的。 （因此，也許這個方法沒有很好地命名？）

該方法在內部檢查組件的對等方是否是LightweightPeer的實例： c.getPeer() instanceof LightweightPeer 。 而且看起來只有NullComponentPeer和NullEmbeddedFramePeer實現了這個接口。 此外， isLightweightComponent的 JavaDoc 有這樣的說法：

如果此組件是輕量級的，即如果它沒有本機窗口系統對等體，則返回 true。

據我所知，peers 是一種獲取原生 OS 事件並將它們路由到組件的幕后手段。

更新

經過進一步調查，我發現在組件可見之前它沒有對等節點（ c.getPeer()返回 null），因此檢查c.getPeer() instanceof LightweightPeer將返回false （似乎表明該組件不是輕量級的是誤導）。 一旦一個組件變得可見，它就會被分配一個對等體（例如，我的測試表明 JLabel 獲取一個 NullComponentPeer 實例作為它的對等體），因此將從對isLightweightComponent的調用返回正確的信息。

總結： isLightweightComponent僅檢查組件的對等方是否是LightweightPeer的實例，並且當它無法實際確定對等方是否為LightweightPeer ，它返回false （而不是返回null或拋出異常）。

Answer 2

的方法（約AWT /擺動GUI系統屬性，尺寸，isXxxXxx等）部分所需可見頂層容器，分別true從isEventDispatchThread ，例如

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

public class Test {

    private JFrame frame = new JFrame();
    private JPanel panel = new JPanel();
    private JLabel label = new JLabel("JLabel");
    private JButton button = new JButton("JButton");
    private String[] list = {"1", "2", "3", "4",};
    private JComboBox comb = new JComboBox(list);
    final JPopupMenu pop = new JPopupMenu();
    private Boolean bol = false;

    public Test() {
        comb.setLightWeightPopupEnabled(true);
        panel.add(label);
        panel.add(button);
        panel.add(comb);
        //
        System.out.println("before visible, out of EDT ---> "
                + (bol = pop.isLightweight()));
        System.out.println("before visible, out of EDT ---> "
                + (bol = pop.isLightweightComponent(comb)));
        pop.setLightWeightPopupEnabled(true);
        pop.add(comb);
        System.out.println("before visible, out of EDT ---> "
                + (bol = comb.isLightweight()));
        System.out.println("before visible, out of EDT ---> "
                + (bol = JComponent.isLightweightComponent(comb)));
        System.out.println("before visible, out of EDT ---> "
                + (bol = button.isLightweight()));
        System.out.println("before visible, out of EDT ---> "
                + (bol = JComponent.isLightweightComponent(button)));
        System.out.println("before visible, out of EDT ---> "
                + (bol = label.isLightweight()));
        System.out.println("before visible, out of EDT ---> "
                + (bol = JComponent.isLightweightComponent(label)));
        //
        frame.add(panel);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 400);
        frame.setVisible(true);
        frame.addMouseListener(new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                System.out.println("mousePressed");
                pop.show(e.getComponent(), e.getX(), e.getY());
                bol = false;
                System.out.println("JComponents are visible, on EDT ---> "
                        + (bol = pop.isLightweight()));
                System.out.println("JComponents are visible, on EDT ---> "
                        + (bol = pop.isLightweightComponent(comb)));
                System.out.println("JComponents are visible, on EDT ---> "
                        + (bol = comb.isLightweight()));
                System.out.println("JComponents are visible, on EDT ---> "
                        + (bol = JComponent.isLightweightComponent(comb)));
                System.out.println("JComponents are visible, on EDT ---> "
                        + (bol = button.isLightweight()));
                System.out.println("JComponents are visible, on EDT ---> "
                        + (bol = JComponent.isLightweightComponent(button)));
                System.out.println("JComponents are visible, on EDT ---> "
                        + (bol = label.isLightweight()));
                System.out.println("JComponents are visible, on EDT ---> "
                        + (bol = JComponent.isLightweightComponent(label)));
            }
        });
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                new Test();
            }
        });
    }
}

帶輸出

run:
before visible, out of EDT ---> false
before visible, out of EDT ---> false
before visible, out of EDT ---> false
before visible, out of EDT ---> false
before visible, out of EDT ---> false
before visible, out of EDT ---> false
before visible, out of EDT ---> false
before visible, out of EDT ---> false
mousePressed
JComponents are visible, on EDT ---> true
JComponents are visible, on EDT ---> true
JComponents are visible, on EDT ---> true
JComponents are visible, on EDT ---> true
JComponents are visible, on EDT ---> true
JComponents are visible, on EDT ---> true
JComponents are visible, on EDT ---> true
JComponents are visible, on EDT ---> true
BUILD SUCCESSFUL (total time: 7 seconds)