Question

我正在尝试做一些事情来追踪问题，但是在paintContents之前我没有什么可以做的，并且通过我的调试器看起来很好看但是我会仔细检查以确保我没有错过任何东西。至少，我想知道如何静默处理这些（例如捕获它们并能够输出有意义的错误消息，因为一旦它被抛出，GUI就会断断续续并冻结一点）。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at sun.java2d.pipe.DuctusShapeRenderer.renderPath(Unknown Source)
        at sun.java2d.pipe.DuctusShapeRenderer.draw(Unknown Source)
        at sun.java2d.pipe.PixelToParallelogramConverter.draw(Unknown Source)
        at sun.java2d.pipe.PixelToParallelogramConverter.draw(Unknown Source)
        at sun.java2d.SunGraphics2D.draw(Unknown Source)
        SNIP - MY CALL TO PAINT THE LAYER
        at com.jhlabs.map.layer.Layer.paintContents(Layer.java:70)
        at com.jhlabs.map.layer.Layer.paint(Layer.java:59)
        at com.jhlabs.map.layer.Layer.paintLayers(Layer.java:76)
        at com.jhlabs.map.layer.Layer.paintContents(Layer.java:68)
        at com.jhlabs.map.layer.Layer.paint(Layer.java:59)
        at com.jhlabs.Globe.paint(Globe.java:305)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintToOffscreen(Unknown Source)
        at javax.swing.BufferStrategyPaintManager.paint(Unknown Source)
        at javax.swing.RepaintManager.paint(Unknown Source)
        at javax.swing.JComponent._paintImmediately(Unknown Source)
        at javax.swing.JComponent.paintImmediately(Unknown Source)
        at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
        at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
        at javax.swing.RepaintManager.seqPaintDirtyRegions(Unknown Source)
        at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(Unknow
n Source)
        at java.awt.event.InvocationEvent.dispatch(Unknown Source)
        at java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.run(Unknown Source)

以下源代码来自Java Map Projection Library。

Layer.paintContents：

public void paintContents(MapGraphics g) {
    if (g != null) {
        paintLayers(g);
        paintFeatures(g);
        paintLayer(g);
    }
}

Layer.paint：

public void paint(MapGraphics g) {
    if (isVisible()) {
        Graphics2D g2d = g.getGraphics2D();
        AffineTransform saveTransform = g2d.getTransform();
        Composite saveComposite = g2d.getComposite();
        Projection saveProjection = g.getProjection();
        Style saveStyle = g.getStyle();
        if (composite != null)
        g2d.setComposite(composite);
        if (transform != null)
        g2d.transform(transform);
        if (style != null)
        g.setStyle(style);
        if (projection != null)
        g.setProjection(projection);
        paintContents(g);
        g.setStyle(saveStyle);
        g.setProjection(saveProjection);
        g2d.setComposite(saveComposite);
        g2d.setTransform(saveTransform);
    }
}

Layer.paintLayers：

public void paintLayers(MapGraphics g) {
for (Iterator<Layer> it = getLayersIterator(); it.hasNext();) {
    Layer l = (Layer) it.next();
    l.paint(g);
}
}

Globe.paint：

public void paint(Graphics g) {
Graphics2D g2 = (Graphics2D) g;

// Turn on antialiasing - otherwise it looks horrible
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

// Put the origin at bottom left
g2.translate(0, getHeight());
g2.scale(1, -1);

// Put the globe in the middle
g2.translate(getWidth() / 2, getHeight() / 2);

Point2D.Float p = new Point2D.Float(1, 0);
transform.deltaTransform(p, p);
float rscale = 1.0f / (float) Math.sqrt(p.x * p.x + p.y * p.y);
g2.setStroke(new BasicStroke(rscale * 0.5f));

MapGraphics mg = MapGraphics.getGraphics(g2, new Rectangle(getSize()));
seaLayer.setVisible(showSea);
tissotLayer.setVisible(showTissot);
worldLayer.setVisible(showWorld);
graticuleLayer.setVisible(showGraticule);
map.paint(mg);

if (showNight) {
    Color c = new Color(0, 0, 0, 0.5f);
    GeneralPath gc = new GeneralPath();
    ProjectionPainter.smallCircle(45, 5, 87, 180, gc, true);
    gc.closePath();
    ProjectionPainter pp = ProjectionPainter.getProjectionPainter(projection);
    pp.drawPath(g2, gc, null, c);

}

}

Answer 1

这意味着在处理事件时（在这种情况下看起来像重绘）会抛出NullPointerException。这意味着你试图在一个对象上调用一个方法，结果证明它是null。

com.jhlabs.map.layer.Layer类的paintContents（）方法中第70行的内容有一个空引用。

Answer 2

检查sun lib的源代码是否可用。如果是这样：指示您在抛出NPE时进行debuger执行。然后你可以检查null的对象并猜测（或回溯）该对象应该在该引用中

是什么导致AWT-EventQueue-0线程中的NullPointerException

2 个答案: