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利用Java怎么實現一個排他鎖
利用Java怎么實現一個排他鎖？很多新手對此不是很清楚，為了幫助大家解決這個難題，下面小編將為大家詳細講解，有這方面需求的人可以來學習下，希望你能有所收獲。
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一 .前言
某年某月某天，同事說需要一個文件排他鎖功能，需求如下：
（1）寫操作是排他屬性
（2）適用于同一進程的多線程／也適用于多進程的排他操作
（3）容錯性：獲得鎖的進程若Crash，不影響到后續進程的正常獲取鎖
二 .解決方案
1. 最初的構想
在Java領域，同進程的多線程排他實現還是較簡易的。比如使用線程同步變量標示是否已鎖狀態便可。但不同進程的排他實現就比較繁瑣。使用已有API，自然想到 java.nio.channels.FileLock：如下
/**
* @param file
* @param strToWrite
* @param append
* @param lockTime 以毫秒為單位,該值只是方便模擬排他鎖時使用，－1表示不考慮該字段
* @return
*/
public static boolean lockAndWrite(File file, String strToWrite, boolean append,int lockTime){
if(!file.exists()){
return false;
}
RandomAccessFile fis = null;
FileChannel fileChannel = null;
FileLock fl = null;
long tsBegin = System.currentTimeMillis();
try {
fis = new RandomAccessFile(file, "rw");
fileChannel = fis.getChannel();
fl = fileChannel.tryLock();
if(fl == null || !fl.isValid()){
return false;
}
log.info("threadId = {} lock success", Thread.currentThread());
// if append
if(append){
long length = fis.length();
fis.seek(length);
fis.writeUTF(strToWrite);
//if not, clear the content , then write
}else{
fis.setLength(0);
fis.writeUTF(strToWrite);
}
long tsEnd = System.currentTimeMillis();
long totalCost = (tsEnd - tsBegin);
if(totalCost < lockTime){
Thread.sleep(lockTime - totalCost);
}
} catch (Exception e) {
log.error("RandomAccessFile error",e);
return false;
}finally{
if(fl != null){
try {
fl.release();
} catch (IOException e) {
e.printStackTrace();
}
}
if(fileChannel != null){
try {
fileChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if(fis != null){
try {
fis.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return true;
} 一切看起來都是那么美好，似乎無懈可擊。于是加上兩種測試場景代碼：
（1）同一進程，兩個線程同時爭奪鎖，暫定命名為測試程序A，期待結果：有一線程獲取鎖失敗
（2）執行兩個進程，也就是執行兩個測試程序A，期待結果：有一進程某線程獲得鎖，另一線程獲取鎖失敗
public static void main(String[] args) {
new Thread("write-thread-1-lock"){
@Override
public void run() {
FileLockUtils.lockAndWrite(new File("/data/hello.txt"), "write-thread-1-lock" ＋ System.currentTimeMillis(), false, 30 * 1000);}
}.start();
new Thread("write-thread-2-lock"){
@Override
public void run() {
FileLockUtils.lockAndWrite(new File("/data/hello.txt"), "write-thread-2-lock" + System.currentTimeMillis(), false, 30 * 1000);
}
}.start();
} 2.世界不像你想的那樣
上面的測試代碼在單個進程內可以達到我們的期待。但是同時運行兩個進程，在Mac環境（java8) 第二個進程也能正常獲取到鎖，在Win7（java7）第二個進程則不能獲取到鎖。為什么？難道TryLock不是排他的？
其實不是TryLock不是排他，而是channel.close 的問題，官方說法：
On some systems, closing a channel releases all locks held by the Java virtual machine on the underlying file regardless of whether the locks were acquired via that channel or via another channel open on the same file.It is strongly recommended that, within a program, a unique channel be used to acquire all locks on any given file.
原因就是在某些操作系統，close某個channel將會導致JVM釋放所有lock。也就是說明了上面的第二個測試用例為什么會失敗，因為第一個進程的第二個線程獲取鎖失敗后，我們調用了channel.close ，所有將會導致釋放所有lock，所有第二個進程將成功獲取到lock。
在經過一段曲折尋找真理的道路后，終于在stackoverflow上找到一個帖子 ，指明了 lucence 的 NativeFSLock，NativeFSLock 也是存在多個進程排他寫的需求。筆者參考的是lucence 4.10.4 的NativeFSLock源碼，具體可見地址，具體可見obtain 方法，NativeFSLock 的設計思想如下：
（1）每一個鎖，都有本地對應的文件。
（2）本地一個static類型線程安全的Set
（3）假設LOCK_HELD 沒有對應文件路徑，則可對File的channel TryLock。
public synchronized boolean obtain() throws IOException {
if (lock != null) {
// Our instance is already locked:
return false;
}
// Ensure that lockDir exists and is a directory.
if (!lockDir.exists()) {
if (!lockDir.mkdirs())
throw new IOException("Cannot create directory: " + lockDir.getAbsolutePath());
} else if (!lockDir.isDirectory()) {
// TODO: NoSuchDirectoryException instead?
throw new IOException("Found regular file where directory expected: " + lockDir.getAbsolutePath());
}
final String canonicalPath = path.getCanonicalPath();
// Make sure nobody else in-process has this lock held
// already, and, mark it held if not:
// This is a pretty crazy workaround for some documented
// but yet awkward JVM behavior:
//
// On some systems, closing a channel releases all locks held by the
// Java virtual machine on the underlying file
// regardless of whether the locks were acquired via that channel or via
// another channel open on the same file.
// It is strongly recommended that, within a program, a unique channel
// be used to acquire all locks on any given
// file.
//
// This essentially means if we close "A" channel for a given file all
// locks might be released... the odd part
// is that we can't re-obtain the lock in the same JVM but from a
// different process if that happens. Nevertheless
// this is super trappy. See LUCENE-5738
boolean obtained = false;
if (LOCK_HELD.add(canonicalPath)) {
try {
channel = FileChannel.open(path.toPath(), StandardOpenOption.CREATE, StandardOpenOption.WRITE);
try {
lock = channel.tryLock();
obtained = lock != null;
} catch (IOException | OverlappingFileLockException e) {
// At least on OS X, we will sometimes get an
// intermittent "Permission Denied" IOException,
// which seems to simply mean "you failed to get
// the lock". But other IOExceptions could be
// "permanent" (eg, locking is not supported via
// the filesystem). So, we record the failure
// reason here; the timeout obtain (usually the
// one calling us) will use this as "root cause"
// if it fails to get the lock.
failureReason = e;
}
} finally {
if (obtained == false) { // not successful - clear up and move
// out
clearLockHeld(path);
final FileChannel toClose = channel;
channel = null;
closeWhileHandlingException(toClose);
}
}
}
return obtained;
} 看完上述內容是否對您有幫助呢？如果還想對相關知識有進一步的了解或閱讀更多相關文章，請關注創新互聯行業資訊頻道，感謝您對創新互聯的支持。
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