FileInputStream NullpointerException

Question

我正在尝试读取属性文件/res/version.num。 但我得到的只是一个 NPE。

try {
    URL url = getClass().getResource("/res/version.num");
    logger.writeLog(JammEnum.INFO, "version.num path: " + url.getPath());
    File prop_file = new File(url.getPath());
    FileInputStream fis = new FileInputStream(prop_file);
    prop.load(fis);
    fis.close();
} 
catch (FileNotFoundException e) {
    logger.writeLog(JammEnum.ERROR, "File not found: version.num");
    e.printStackTrace();
} 
catch (IOException e) {
    logger.writeLog(JammEnum.ERROR, "Cannot read version.num");
    e.printStackTrace();
}

输出是：

(II) - version.num path: /home/andreas/Programmieren/Java/Jamm/bin/res/version.num
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at jamm.gui.InfoGui.<init>(InfoGui.java:83)
    at jamm.gui.FinanzmanagerGui$ButtonInfoListener.actionPerformed(FinanzmanagerGui.java:553)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6504)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at java.awt.Component.processEvent(Component.java:6269)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4860)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4686)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2713)
    at java.awt.Component.dispatchEvent(Component.java:4686)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
    at java.awt.EventQueue.access$000(EventQueue.java:101)
    at java.awt.EventQueue$3.run(EventQueue.java:666)
    at java.awt.EventQueue$3.run(EventQueue.java:664)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:680)
    at java.awt.EventQueue$4.run(EventQueue.java:678)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

第 83 行是：FileInputStream fis = new FileInputStream(prop_file);

文件的路径记录正确。但是为什么我会得到这个异常？

Answer 1

试试这个例子。

$ cat src/PrintVersion.java 
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;

/**
 * @author peter.lawrey
 */
public class PrintVersion {
    public static void main(String... args) throws IOException {
        InputStream is = PrintVersion.class.getClassLoader().getResourceAsStream("res/version.num");
        String version = asString(is);
        System.out.println("Version: "+version);
    }

    public static String asString(InputStream is) throws IOException {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        byte[] bytes = new byte[1024];
        for(int len; (len = is.read(bytes))>0;)
            baos.write(bytes, 0, len);
        return baos.toString();
    }
}
$ cat src/res/version.num 
version 1.0
$ javac src/PrintVersion.java
$ ls src
PrintVersion.class  PrintVersion.java  res
$ java -cp src PrintVersion
Version: version 1.0

---

url 为空，因为它找不到资源。即使它不为空，这也不是获取文件的正确方法，因为它仅在文件位于文件系统上时才有效。

你应该试试

InputStream is = getClass().getClassLoader()
                 .getResourceAsInputStream("/res/version.num");

Answer 2

您需要使用ClassLoader.getResourceAsStream()，而不是 FileInputStream。