java中BitCaskLock的使用方法

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序

本文主要研究一下BitCaskLock

BitCaskLock

bitcask-java/src/main/java/com/trifork/bitcask/BitCaskLock.java

public class BitCaskLock {        public static enum Stale {                OK, NOT_STALE        }        public static enum Type {                WRITE, MERGE;                private  String type_name() {                        if (this == MERGE) return "merge";                        if (this == WRITE) return "write";                        throw new RuntimeException();                }        }        private RandomAccessFile file;        private boolean is_write_lock;        private File filename;        private BitCaskLock(RandomAccessFile file, File filename, boolean isWriteLock) {                this.file = file;                this.filename = filename;                this.is_write_lock = isWriteLock;        }        public static BitCaskLock acquire(Type type, File dirname) throws IOException {                File lock_filename = lock_filename(type, dirname);                foo: do {                        try {                                BitCaskLock lock = lock_acquire(lock_filename, true);                                String lock_contents = Integer.toString(OS.getpid()) + " \n";                                lock.write_data(ByteString.copyFromUtf8(lock_contents));                                return lock;                        } catch (FileAlreadyExistsException e) {                                delete_stale_lock(lock_filename);                                continue foo;                        }                } while (false);                return null;        }        public void release() throws IOException {                if (file != null) {                        if (is_write_lock) {                                file.close();                                filename.delete();                        }                }                file = null;        }        //......}       

• BitCaskLock的acquire方法，先通过lock_acquire获取BitCaskLock，如果出现FileAlreadyExistsException则执行delete_stale_lock；其release方法针对is_write_lock执行file.close()及filename.delete()

lock_acquire

bitcask-java/src/main/java/com/trifork/bitcask/BitCaskLock.java

      private static BitCaskLock lock_acquire(File lockFilename, boolean is_write_lock)                        throws IOException {                if (is_write_lock) {                        if (lockFilename.createNewFile() == false) {                                // file already exists, so we fail!                                throw new FileAlreadyExistsException(lockFilename);                        }                }                RandomAccessFile f = new RandomAccessFile(lockFilename,                                is_write_lock ? "rws" : "r");                return new BitCaskLock(f, lockFilename, is_write_lock);        }

• lock_acquire方法针对is_write_lock，执行lockFilename.createNewFile()，若为false则抛出FileAlreadyExistsException

delete_stale_lock

bitcask-java/src/main/java/com/trifork/bitcask/BitCaskLock.java

      public static Stale delete_stale_lock(Type type, File dirname) throws IOException {                return delete_stale_lock(lock_filename(type, dirname));        }                private static Stale delete_stale_lock(File lockFilename) throws IOException {                BitCaskLock l = null;                try {                        l = lock_acquire(lockFilename, false);                } catch (FileNotFoundException e) {                        return Stale.OK;                } catch (IOException e) {                        return Stale.NOT_STALE;                }                try {                        int pid = l.read_lock_data_pid();                        if (OS.pid_exists(pid)) {                                return Stale.NOT_STALE;                        } else {                                lockFilename.delete();                                return Stale.OK;                        }                } catch (IOException e) {                        return Stale.NOT_STALE;                } finally {                        l.release();                }        }

• delete_stale_lock方法先执行lock_acquire，若出现FileNotFoundException，则返回Stale.OK，若出现IOException则返回Stale.NOT_STALE；否则通过l.read_lock_data_pid()获取pid，若OS.pid_exists(pid)则返回Stale.NOT_STALE，否则执行lockFilename.delete()，返回Stale.OK；最后执行l.release()

小结

BitCaskLock的acquire方法，先通过lock_acquire获取BitCaskLock，如果出现FileAlreadyExistsException则执行delete_stale_lock；其release方法针对is_write_lock执行file.close()及filename.delete()

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