How do I lock database records from Java?

Question

I'm developing a database program in Java with dbf. I need to know how to lock the database records from the Java side.

Example: we have a database table cheques with 5 records in the table (record 1 through 5). There are 2 users, user-1 and user-2. user-1 accesses record 1 and user-2 tries to access record 1 at the same time. I want to lock record 1 to prevent access to that record by user-2 while user-1 is accessing it. How do I do this in Java?

Answer 1

In case of MySQL, you can use, SELECT FOR UPDATE statement for locking the records. Lock will not be released until the transaction completes or rolls back. More on the same here.

Answer 2

It depends on the environment you are working on. for a container managed transaction your container manages the transactions for you and all you have to do is to set the Lockmode to lockmode.write. What this does is that it blocks all write access to the class methods while userA is accessing record 1. On the other hand for a stand alone application you can just add Synchronization key word to your method to control concurrent access. I hope this helps.

Answer 3

Not every database supports per-record locking.

Generally, if you are in EE environment, you can use JPA EntityManager#find() method to lock certain record.

Full usage will be like that

// EntityManager em;
YourClass obj = em.find(YourClass.class, primaryKey, LockModeType.WRITE);
// do something
em.merge(obj);

After transaction commit the record(s) will be released.

In non-EE environment, as Darshan Mehta said, connection.createStatement().execute("SELECT * FROM table FOR UPDATE") will be the solution.