可能发生的另一种极端情况：如果您通过读取/写入一个JAR文件，URL然后再尝试删除同一JVM会话中的同一文件。
File f = new File("/tmp/foo.jar");
URL j = f.toURI().toURL();
URL u = new URL("jar:" + j + "!/META-INF/MANIFEST.MF");
URLConnection c = u.openConnection();
// open a Jar entry in auto-closing manner
try (InputStream i = c.getInputStream()) {
// just read some stuff; for demonstration purposes only
byte[] first16 = new byte[16];
i.read(first16);
System.out.println(new String(first16));
}
// ...
// i is now closed, so we should be good to delete the jar; but...
System.out.println(f.delete()); // says false!
原因是Java的内部JAR文件处理逻辑倾向于缓存JarFile条目：
// inner class of `JarURLConnection` that wraps the actual stream returned by `getInputStream()`
class JarURLInputStream extends FilterInputStream {
JarURLInputStream(InputStream var2) {
super(var2);
}
public void close() throws IOException {
try {
super.close();
} finally {
// if `getUseCaches()` is set, `jarFile` won't get closed!
if (!JarURLConnection.this.getUseCaches()) {
JarURLConnection.this.jarFile.close();
}
}
}
}
从构造之时直到调用它为止JarFile，每个（而是底层ZipFile结构）都将持有文件的句柄close()：
public ZipFile(File file, int mode, Charset charset) throws IOException {
// ...
jzfile = open(name, mode, file.lastModified(), usemmap);
// ...
}
// ...
private static native long open(String name, int mode, long lastModified,
boolean usemmap) throws IOException;
关于此NetBeans问题，有很好的解释。
显然，有两种方法可以“解决”此问题：
您可以为当前JVM URLConnection或URLConnection当前JVM会话中的所有future （全局）禁用JAR文件缓存：
URL u = new URL("jar:" + j + "!/META-INF/MANIFEST.MF");
URLConnection c = u.openConnection();
// for only c
c.setUseCaches(false);
// globally; for some reason this method is not static,
// so we still need to access it through a URLConnection instance :(
c.setDefaultUseCaches(false);
[HACK WARNING！]完成后，您可以JarFile从缓存中手动清除它。缓存管理器sun.net.www.protocol.jar.JarFileFactory是软件包专用的，但是一些反射魔术可以为您完成任务：
class JarBridge {
static void closeJar(URL url) throws Exception {
// JarFileFactory jarFactory = JarFileFactory.getInstance();
Class<?> jarFactoryClazz = Class.forName("sun.net.www.protocol.jar.JarFileFactory");
Method getInstance = jarFactoryClazz.getMethod("getInstance");
getInstance.setAccessible(true);
Object jarFactory = getInstance.invoke(jarFactoryClazz);
// JarFile jarFile = jarFactory.get(url);
Method get = jarFactoryClazz.getMethod("get", URL.class);
get.setAccessible(true);
Object jarFile = get.invoke(jarFactory, url);
// jarFactory.close(jarFile);
Method close = jarFactoryClazz.getMethod("close", JarFile.class);
close.setAccessible(true);
//noinspection JavaReflectionInvocation
close.invoke(jarFactory, jarFile);
// jarFile.close();
((JarFile) jarFile).close();
}
}
// and in your code:
// i is now closed, so we should be good to delete the jar
JarBridge.closeJar(j);
System.out.println(f.delete()); // says true, phew.
请注意：所有这些都是基于Java 8代码库（1.8.0_144）; 它们可能不适用于其他/更高版本。