blocks|key|1529380|text|将java.nio.channels.FileLock与java.nio.channels.FileChannel结合使用|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1529381|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/javase/6/docs/api/java/nio/channels/FileLock.html|1|http://java.sun.com/javase/6/docs/api/java/nio/channels/FileChannel.html^0|1|Q|0|S|T|1|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]|$A|R|B|S|1|T]]|C|$]]|$1|D|3|-4|5|6|7|U|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]|L|$5|G|H|I|C|$J|M]]]]

use <a href="http://java.sun.com/javase/6/docs/api/java/nio/channels/FileLock.html" rel="noreferrer">java.nio.channels.FileLock</a> in conjunction with <a href="http://java.sun.com/javase/6/docs/api/java/nio/channels/FileChannel.html" rel="noreferrer">java.nio.channels.FileChannel</a>

blocks|key|1529452|text|FileChannel.lock可能就是您想要的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1529453|try+(
++++FileInputStream+in+=+new+FileInputStream(file);
++++java.nio.channels.FileLock+lock+=+in.getChannel().lock();
++++Reader+reader+=+new+InputStreamReader(in,+charset)
)+{
++++...
}|code-block|syntax|javascript|1529454|(免责声明:代码没有编译，当然也没有经过测试。)|1529455|请注意API+doc+for+FileLock中标题为“平台依赖项”的部分。|offset|length|1529456|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/nio/channels/FileLock.html#pdep^0|0|0|0|3|K|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|X|8|@]|9|@$K|Y|L|Z|1|10]]|A|$]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

FileChannel.lock is probably what you want.

<pre><code>try (
 FileInputStream in = new FileInputStream(file);
 java.nio.channels.FileLock lock = in.getChannel().lock();
 Reader reader = new InputStreamReader(in, charset)
) {
 ...
}
</code></pre>

(Disclaimer: Code not compiled and certainly not tested.)

Note the section entitled "platform dependencies" in the <a href="https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/nio/channels/FileLock.html#pdep" rel="noreferrer">API doc for FileLock</a>.

blocks|key|1744470|text|使用锁，获取它的通道，然后调用RandomAccessFile+()。输入流或输出流提供的通道没有足够的权限来正确锁定。一定要在finally块中调用unlock()+(关闭文件不一定会释放锁)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1744471|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Use a RandomAccessFile, get it's channel, then call lock(). The channel provided by input or output streams does not have sufficient privileges to lock properly. Be sure to call unlock() in the finally block (closing the file doesn't necessarily release the lock).

blocks|key|1744553|text|这可能不是你正在寻找的东西，但为了从另一个角度遇到问题……|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1744554|这两个Java进程可能想要访问同一应用程序中的同一文件吗？也许您可以通过一个同步的方法(或者更好的是，使用JSR-166)来过滤对文件的所有访问？这样，您就可以控制对文件的访问，甚至可以对访问请求进行排队。|offset|length|1744555|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/package-summary.html^0|0|1H|7|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

This may not be what you are looking for, but in the interest of coming at a problem from another angle....

Are these two Java processes that might want to access the same file in the same application? Perhaps you can just filter all access to the file through a single, synchronized method (or, even better, using <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/package-summary.html" rel="noreferrer">JSR-166</a>)? That way, you can control access to the file, and perhaps even queue access requests.

blocks|key|1529664|text|不要使用java.io包中的类，而要使用java.nio包。后者有一个FileLock类。您可以将锁定应用于FileChannel。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1529665|+try+{
++++++++//+Get+a+file+channel+for+the+file
++++++++File+file+=+new+File("filename");
++++++++FileChannel+channel+=+new+RandomAccessFile(file,+"rw").getChannel();

++++++++//+Use+the+file+channel+to+create+a+lock+on+the+file.
++++++++//+This+method+blocks+until+it+can+retrieve+the+lock.
++++++++FileLock+lock+=+channel.lock();

++++++++/*
+++++++++++use+channel.lock+OR+channel.tryLock();
++++++++*/

++++++++//+Try+acquiring+the+lock+without+blocking.+This+method+returns
++++++++//+null+or+throws+an+exception+if+the+file+is+already+locked.
++++++++try+{
++++++++++++lock+=+channel.tryLock();
++++++++}+catch+(OverlappingFileLockException+e)+{
++++++++++++//+File+is+already+locked+in+this+thread+or+virtual+machine
++++++++}

++++++++//+Release+the+lock+-+if+it+is+not+null!
++++++++if(+lock+!=+null+)+{
++++++++++++lock.release();
++++++++}

++++++++//+Close+the+file
++++++++channel.close();
++++}+catch+(Exception+e)+{
++++}|code-block|syntax|javascript|1529666|entityMap^0|4|7|K|8|Z|8|1I|B|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|L|$]]

Don't use the classes in the<code>java.io</code> package, instead use the <code>java.nio</code> package . The latter has a <code>FileLock</code> class. You can apply a lock to a <code>FileChannel</code>.

<pre><code> try {
 // Get a file channel for the file
 File file = new File("filename");
 FileChannel channel = new RandomAccessFile(file, "rw").getChannel();

 // Use the file channel to create a lock on the file.
 // This method blocks until it can retrieve the lock.
 FileLock lock = channel.lock();

 /*
 use channel.lock OR channel.tryLock();
 */

 // Try acquiring the lock without blocking. This method returns
 // null or throws an exception if the file is already locked.
 try {
 lock = channel.tryLock();
 } catch (OverlappingFileLockException e) {
 // File is already locked in this thread or virtual machine
 }

 // Release the lock - if it is not null!
 if( lock != null ) {
 lock.release();
 }

 // Close the file
 channel.close();
 } catch (Exception e) {
 }
</code></pre>

blocks|key|1744670|text|下面是一个示例代码片段，用于锁定文件，直到JVM完成该文件的处理。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1744671|+public+static+void+main(String[]+args)+throws+InterruptedException+{
++++File+file+=+new+File(FILE_FULL_PATH_NAME);
++++RandomAccessFile+in+=+null;
++++try+{
++++++++in+=+new+RandomAccessFile(file,+"rw");
++++++++FileLock+lock+=+in.getChannel().lock();
++++++++try+{

++++++++++++while+(in.read()+!=+-1)+{
++++++++++++++++System.out.println(in.readLine());
++++++++++++}
++++++++}+finally+{
++++++++++++lock.release();
++++++++}
++++}+catch+(FileNotFoundException+e)+{
++++++++e.printStackTrace();
++++}+catch+(IOException+e)+{
++++++++e.printStackTrace();
++++}finally+{
++++++++try+{
++++++++++++in.close();
++++++++}+catch+(IOException+e)+{
++++++++++++e.printStackTrace();
++++++++}
++++}

}|code-block|syntax|javascript|1744672|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Below is a sample snippet code to lock a file until it's process is done by JVM. 

<pre><code> public static void main(String[] args) throws InterruptedException {
 File file = new File(FILE_FULL_PATH_NAME);
 RandomAccessFile in = null;
 try {
 in = new RandomAccessFile(file, "rw");
 FileLock lock = in.getChannel().lock();
 try {

 while (in.read() != -1) {
 System.out.println(in.readLine());
 }
 } finally {
 lock.release();
 }
 } catch (FileNotFoundException e) {
 e.printStackTrace();
 } catch (IOException e) {
 e.printStackTrace();
 }finally {
 try {
 in.close();
 } catch (IOException e) {
 e.printStackTrace();
 }
 }

}
</code></pre>

blocks|key|1744695|text|如果使用winscp或ftp进行传输，请对unix使用此选项：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1744696|public+static+void+isFileReady(File+entry)+throws+Exception+{
++++++++long+realFileSize+=+entry.length();
++++++++long+currentFileSize+=+0;
++++++++do+{
++++++++++++try+(FileInputStream+fis+=+new+FileInputStream(entry);)+{
++++++++++++++++currentFileSize+=+0;
++++++++++++++++while+(fis.available()+>+0)+{
++++++++++++++++++++byte[]+b+=+new+byte[1024];
++++++++++++++++++++int+nResult+=+fis.read(b);
++++++++++++++++++++currentFileSize+%2B=+nResult;
++++++++++++++++++++if+(nResult+==+-1)
++++++++++++++++++++++++break;
++++++++++++++++}
++++++++++++}+catch+(Exception+e)+{
++++++++++++++++e.printStackTrace();
++++++++++++}
++++++++++++System.out.println("currentFileSize="+%2B+currentFileSize+%2B+",+realFileSize="+%2B+realFileSize);
++++++++}+while+(currentFileSize+!=+realFileSize);
++++}|code-block|syntax|javascript|1744697|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Use this for unix if you are transferring using winscp or ftp:
<pre><code>public static void isFileReady(File entry) throws Exception {
 long realFileSize = entry.length();
 long currentFileSize = 0;
 do {
 try (FileInputStream fis = new FileInputStream(entry);) {
 currentFileSize = 0;
 while (fis.available() &gt; 0) {
 byte[] b = new byte[1024];
 int nResult = fis.read(b);
 currentFileSize += nResult;
 if (nResult == -1)
 break;
 }
 } catch (Exception e) {
 e.printStackTrace();
 }
 System.out.println(&quot;currentFileSize=&quot; + currentFileSize + &quot;, realFileSize=&quot; + realFileSize);
 } while (currentFileSize != realFileSize);
 }
</code></pre>

I have a Java process that opens a file using a FileReader. How can I prevent another (Java) process from opening this file, or at least notify that second process that the file is already opened? Does this automatically make the second process get an exception if the file is open (which solves my problem) or do I have to explicitly open it in the first process with some sort of flag or argument?
<h3>To clarify:</h3>
I have a Java app that lists a folder and opens each file in the listing for processing it. It processes each file after the other. The processing of each file consists of reading it and doing some calculations based on the contents and it takes about 2 minutes. I also have another Java app that does the same thing but instead writes on the file. What I want is to be able to run these apps at the same time so the scenario goes like this. ReadApp lists the folder and finds files A, B, C. It opens file A and starts the reading. WriteApp lists the folder and finds files A, B, C. It opens file A, sees that is is open (by an exception or whatever way) and goes to file B. ReadApp finishes file A and continues to B. It sees that it is open and continues to C. It is crucial that WriteApp doesn't write while ReadApp is reading the same file or vice versa. They are different processes.

How can I lock a file using java (if possible)

Java

我有一个使用FileReader打开文件的Java进程。如何防止另一个(Java)进程打开该文件，或者至少通知第二个进程该文件已经打开？如果文件是打开的(这解决了我的问题)，这是否会自动使第二个进程获得异常，还是必须在第一个进程中使用某种标志或参数显式打开它？澄清一下：我有一个Java应用程序，它列出了一个文件夹，并打...

问如何使用java锁定文件(如果可能)
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