Use of dbms_lock inconsistent using jdbc - multithreading

Question

Why does the protected Java code execute in more then one thread at a time?
Thread-1 - protected code iteration 0
Thread-0 - protected code iteration 0
Thread-1 - protected code iteration 1
Thread-0 - protected code iteration 1
Thread-0 - protected code iteration 2
Thread-1 - protected code iteration 2
Thread-1 - protected code iteration 3
Thread-0 - protected code iteration 3
Run it enough times it can execute as expected.
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();
}
}
}