问题描述
使用 ImageIO.write() API 调用，当我传递一个不存在的路径，如 "\\abc\abc.png" 时，我得到 NullPointerException.我故意传递不存在的路径来测试某些东西，但我得到的不是 FileNotFoundException，而是 NPE.这是为什么?
With ImageIO.write() API call, I get NullPointerException when I pass a non-existent path like "\\abc\abc.png". I pass the non-existent path purposely to test something but instead of getting FileNotFoundException, I get NPE. Why is that?
ImageIO.write() API 应该抛出 IOException 但不知道为什么我得到 NPE.
ImageIO.write() API is supposed to throw IOException but don't why I get NPE.
我使用异常消息字符串在消息框中向用户显示它，但在这种情况下 NPE.getLocalizedMessage() 返回空字符串，因此弹出窗口是空的，上面只有一个图标.
I use exception message string to show it in a message box to user but in this case NPE.getLocalizedMessage() returns empty string and hence the popup is empty with just an icon on it.
推荐答案
他是是对的.例如，这段代码:
He is right, though. For example, this code:
public static void main(String[] args) throws IOException {
BufferedImage image = new BufferedImage(32, 32, BufferedImage.TYPE_INT_ARGB);
File out = new File("\\\\ABC\\abc.png");
ImageIO.write(image, "png", out);
}
给予
java.io.FileNotFoundException: \\ABC\abc.png (The network path was not found)
at java.io.RandomAccessFile.open(Native Method)
at java.io.RandomAccessFile.<init>(RandomAccessFile.java:233)
at javax.imageio.stream.FileImageOutputStream.<init>(FileImageOutputStream.java:69)
at com.sun.imageio.spi.FileImageOutputStreamSpi.createOutputStreamInstance(FileImageOutputStreamSpi.java:55)
at javax.imageio.ImageIO.createImageOutputStream(ImageIO.java:419)
at javax.imageio.ImageIO.write(ImageIO.java:1530)
at javaapplication145.JavaApplication145.main(JavaApplication145.java:24)
Exception in thread "main" java.lang.NullPointerException
at javax.imageio.ImageIO.write(ImageIO.java:1538)
at javaapplication145.JavaApplication145.main(JavaApplication145.java:24)
原因是 FileImageOutputStreamSpi.createOutputStreamInstance 吞下了 FileNotFoundException 然后当 ImageIO.write 尝试关闭未打开的流时 NPE 出现.
The reason is that FileImageOutputStreamSpi.createOutputStreamInstance swallows the FileNotFoundException and then the NPE comes when ImageIO.write tries to close a stream that didn't open.
为什么异常被如此残酷地压制，我不知道.代码片段是
Why the exception is suppressed so brutally, I don't know. The code fragment is
try {
return new FileImageOutputStream((File)output);
} catch (Exception e) {
e.printStackTrace();
return null;
}
唯一的解决方案是在尝试使用 ImageIO 之前验证路径.
The only solution is to verify the path before attempting to use ImageIO.
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