关于java:聊聊BitCaskLock

序

本文次要钻研一下 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 办法，先通过 lock_acquire 获取 BitCaskLock，如果呈现 FileAlreadyExistsException 则执行 delete_stale_lock；其 release 办法针对 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 办法针对 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 办法先执行 lock_acquire，若呈现 FileNotFoundException，则返回 Stale.OK，若呈现 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；最初执行 l.release()

小结

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

doc

• BitCaskLock