使用jdbc使用dbms_lock不一致

Question

为什么受保护的Java代码一次只能在多个线程中执行？ Thread-1 - 受保护的代码迭代 0
Thread-0 - 受保护的代码迭代 0
Thread-1 - 受保护的代码迭代 1
Thread-0 - 受保护的代码迭代 1
Thread-0 - 受保护的代码迭代 2
Thread-1 - 受保护的代码迭代 2
Thread-1 - 受保护的代码迭代 3
Thread-0 - 受保护的代码迭代 3
运行足够多次，它可以按预期执行。

public class DbmsLockTest implements Runnable {
Connection con; String key; int timeout;

public DbmsLockTest(Connection con, String key, int timeout) {
    this.con = con; this.key = key; this.timeout = timeout;
}

public static void log(String str) {
    System.out.println(new Date() + " - "
            + Thread.currentThread().getName() + " - " + str);
}

@Override
public void run() {
    lockKey(con, key, timeout);
}

public static void lockKey(Connection con, String key, int timeout) {
    log("start lockKey " + "  key: " + key);

    CallableStatement cStmt = null;
    int rc = 0;
    try {
        StringBuilder sql = new StringBuilder(500);
        sql.append("DECLARE");
        sql.append("   v_lockhandle VARCHAR2(200);");
        sql.append("BEGIN");
        sql.append("   dbms_lock.allocate_unique(lockname    => ?,  lockhandle  => v_lockhandle);");
        sql.append("   ? := dbms_lock.request(lockhandle  => v_lockhandle, lockmode        => 6,");
        sql.append("                          timeout         => ?, release_on_commit   =>true);");
        sql.append("   ? := v_lockhandle;");
        sql.append("END;");

        String lockKey = "LockKey-" + key;

        cStmt = con.prepareCall(sql.toString());
        cStmt.setString(1, lockKey);
        cStmt.registerOutParameter(2, Types.NUMERIC);
        cStmt.setInt(3, timeout);
        cStmt.registerOutParameter(4, Types.VARCHAR);

        log("executeUpdate start: " + lockKey + "] ");
        cStmt.executeUpdate();
        log("executeUpdate end: " + lockKey + "] ");

        rc = cStmt.getInt(2);
        log("return value from request=[" + rc + "] ");

        if (rc != 0) {
            System.out.println("6001 lock obtained: "
                    + Thread.currentThread().getName());
            throw new RuntimeException("lock acquisition failed with code=" + rc);
        }

        log("v_lockhandle=[" + cStmt.getString(4) + "] ");

        for (int i = 0; i < 4; i++) {
            try {
                Thread.sleep(1000 * 1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            log("***** protected code iteration ***** " + i);
        }
        con.commit();

    } catch (SQLException e) {
        log("int timeout: " + Thread.currentThread().getName());
        throw new RuntimeException("SQLException locking balance for user "
                + key, e);
    } finally {
        try {
            if (cStmt != null) {
                cStmt.close();
            }
            if (con != null) {
                con.close();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    log("Exiting=[" + rc + "] ");
}

public static void main(String[] args) throws Exception {
    new oracle.jdbc.OracleDriver();
    List<Thread> list = new ArrayList<Thread>();
    for (int i = 0; i < 2; i++) {
        Connection connection = DriverManager
                .getConnection("jdbc:oracle:thin:@localhost:1521:xe",
                        "<username>", "password");
        Thread t = new Thread(new DbmsLockTest(connection, "mykey", 10));
        list.add(t);
    }

    for (Thread t : list) {
        t.start();
    }
}
}