java - 随机访问文件 FileLock : java. io 与 java.nio

Question

我注意到 java.io和 java.nio随机访问文件的实现在 FileLocks 方面略有不同。得到处理。

似乎(在 Windows 上)java.io 为您提供强制文件锁定，而 java.nio 分别在请求时为您提供建议文件锁定。强制文件锁意味着锁适用于所有进程，建议适用于遵循相同锁定协议(protocol)的行为良好的进程。

如果我运行以下示例，我可以手动删除*.nio 文件，而*.io 文件拒绝删除。

import java.io.*;
import java.lang.management.ManagementFactory;
import java.nio.*;
import java.nio.channels.*;
import java.nio.file.*;

public class NioIoLock {

    public static void main(String[] args) throws IOException, InterruptedException {
        String workDir = System.getProperty("user.dir");

        FileChannel channelIo, channelNio;
        FileLock lockIo, lockNio;

        // use io
        {
            String fileName = workDir
                    + File.separator
                    + ManagementFactory.getRuntimeMXBean().getName()
                    + ".io";
            File lockFile = new File(fileName);
            lockFile.deleteOnExit();
            RandomAccessFile file = new RandomAccessFile(lockFile, "rw");            

            channelIo = file.getChannel();
            lockIo = channelIo.tryLock();
            if (lockIo != null) {                   
                channelIo.write(ByteBuffer.wrap("foobar".getBytes("UTF-8")));
            }

        }

        // use nio
        {
            Path workDirPath = Paths.get(workDir);
            Path file = workDirPath.resolve(
                    Paths.get(ManagementFactory.getRuntimeMXBean().getName() + ".nio"));

            // open/create test file
            channelNio = FileChannel.open(
                    file, StandardOpenOption.READ, StandardOpenOption.WRITE,
                    StandardOpenOption.CREATE, StandardOpenOption.DELETE_ON_CLOSE);

            // lock file
            lockNio = channelNio.tryLock();
            if (lockNio != null) {
                channelNio.write(ByteBuffer.wrap("foobar".getBytes("UTF-8")));
            }

        }

        // do not release locks for some time
        Thread.sleep(10000);

        // release io lock and channel
        if (lockIo != null && lockIo.isValid()) {
            lockIo.release();
        }
        channelIo.close();

        // release nio lock and channel
        if (lockNio != null && lockNio.isValid()) {
            lockNio.release();
        }
        channelNio.close();
    }

}

这是有原因的吗？这两者是否被视为替代品，或者它们是否意味着无限期共存？

Answer 1

TL;DR: 这与锁无关，与文件打开方式有关。您在 io 中看到的是在 Windows 2000 之前 Windows 可以做的最好的事情，即使文件以只读方式打开它也能做到这一点——无法删除该文件。您在 nio 中看到的是使用自 Windows 2000 以来引入的新功能的改进，但如果您愿意，您仍然可以在 nio 中使用旧行为。决定不将该功能移植到 io 的功能中。

全文:我删除了所有与锁定(见下文)以及写入文件相关的代码，它的工作原理与您的代码完全相同；但是，我还发现，如果您在打开“nio” channel 时指定 ExtendedOpenOption.NOSHARE_DELETE，那么当您尝试删除任何两个文件时的行为是相同的(在代码中取消注释并尝试)。还找到了 this question，它有一些解释，不确定是否有帮助。

import java.io.*;
import java.lang.management.ManagementFactory;
import java.nio.*;
import java.nio.channels.*;
import java.nio.file.*;

public class NioIoLock {

    public static void main(String[] args)
            throws IOException, InterruptedException {
        String workDir = System.getProperty("user.dir");

        FileChannel channelIo, channelNio;
        FileLock lockIo, lockNio;

        // use io
        {
            String fileName = workDir + File.separator
                + ManagementFactory.getRuntimeMXBean().getName() + ".io";
            File lockFile = new File(fileName);
            lockFile.deleteOnExit();
            RandomAccessFile file = new RandomAccessFile(lockFile, "rw");
            channelIo = file.getChannel();
        }

        // use nio
        {
            Path workDirPath = Paths.get(workDir);
            Path file = workDirPath.resolve(Paths
                .get(ManagementFactory.getRuntimeMXBean().getName() + ".nio"));

            // open/create test file
            channelNio = FileChannel.open(file, StandardOpenOption.READ,
                StandardOpenOption.WRITE, StandardOpenOption.CREATE,
                StandardOpenOption.DELETE_ON_CLOSE
                /*, com.sun.nio.file.ExtendedOpenOption.NOSHARE_DELETE*/);
        }

        // do not release locks for some time
        Thread.sleep(10000);
    }
}

更新 1: 实际上，我仍然不知道这背后的基本原理，但机制很清楚:RandomAccessFile 构造函数最终从 method winFileHandleOpen 调用以下 native 代码io_util_md.c:

FD
winFileHandleOpen(JNIEnv *env, jstring path, int flags)
{
    ...
    const DWORD sharing =
        FILE_SHARE_READ | FILE_SHARE_WRITE;
    ... // "sharing" not updated anymore
    h = CreateFileW(
        pathbuf,            /* Wide char path name */
        access,             /* Read and/or write permission */
        sharing,            /* File sharing flags */
        NULL,               /* Security attributes */
        disposition,        /* creation disposition */
        flagsAndAttributes, /* flags and attributes */
        NULL);
    ...
}

因此，FILE_SHARE_DELETE 标志未设置，您无法设置它。另一方面，调用 java.nio.channels.FileChannel.open(Path, OpenOption...) eventually 会考虑这个标志:

    private static FileDescriptor open(String pathForWindows,
                                       String pathToCheck,
                                       Flags flags,
                                       long pSecurityDescriptor)
        throws WindowsException
    {
        ...
        int dwShareMode = 0;
        if (flags.shareRead)
            dwShareMode |= FILE_SHARE_READ;
        if (flags.shareWrite)
            dwShareMode |= FILE_SHARE_WRITE;
        if (flags.shareDelete)
            dwShareMode |= FILE_SHARE_DELETE;
        ...
        // open file
        long handle = CreateFile(pathForWindows,
                                 dwDesiredAccess,
                                 dwShareMode,
                                 pSecurityDescriptor,
                                 dwCreationDisposition,
                                 dwFlagsAndAttributes);
        ...
    }

更新 2: 从 JDK-6607535 开始:

The suggestion to change the sharing mode is RFE 6357433. It would be great to do that but it may break existing applications (since the current behavior has existed since jdk1.0). Adding a special mode to RandomAccessFile to work around a Windows issues is probably not the right approach either. Also, there is some suggestion that this can help with the issue of deleting files that have a file mapping. This is not so, as a file mapping still prevents a file from being deleted. Anyway, for jdk7 we are working on a new file system API that will address some of the requirements here. The Windows implementation does the right thing so that opened files can be deleted.

另请参阅从那里引用的 JDK-6357433:

A customer has pointed out on the java.net forums that opening a file on Windows using a FileInputStream causes other processes to be unable to delete the file. This is intended behavior as the code is currently written, as the sharing flags specified during the opening of the file are FILE_SHARE_READ and FILE_SHARE_WRITE. See io_util_md.c, fileOpen. However, Windows 2000 supplies a new flag, FILE_SHARE_DELETE, which allows another process to delete the file while it is still open.

最后

WORK AROUND

For jdk7, the workaround is to use new file system API. So instead of new FileInputStream(f) and new FileOutputStream(f), use f.toPath().newInputStream() and f.toPath().newOutputStream().