另一种可能发生这种情况的极端情况：如果您通过 URL 读取/写入 JAR 文件，然后尝试在同一个 JVM 会话中删除同一个文件。
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内部的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();
}
}
}
}
并且每个JarFile（更确切地说，底层ZipFile结构）都将持有文件的句柄，从构建到close()被调用：
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;
this NetBeans issue上有很好的解释。
显然有两种方法可以“解决”这个问题：
-
您可以禁用 JAR 文件缓存 - 为当前的 URLConnection 或当前 JVM 会话中所有未来的 URLConnections（全局）：
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，您可以手动从缓存中清除 JarFile。缓存管理器sun.net.www.protocol.jar.JarFileFactory 是包私有的，但一些反射魔法可以为您完成工作：
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。