J and I and Me
2012-02-06
  Dangerous Code Example in EJB 3.1 Spec
I am currently working on a training concerning Java EE Best Practices. For that reason I am reading through quite some material about Java EE and EJB. However, looking at the EJB 3.1 spec I realized that the code examples are an example of rather bad coding practice. As an example here is the original code from EJB 3.1 Spec p. 348. It is supposed to show how UserTransactions can be used:
@Stateless
@TransactionManagement(BEAN)
public class MySessionBean implements MySession {

  @Resource
  javax.transaction.UserTransaction ut;
  @Resource
  javax.sql.DataSource database1;
  @Resource
  javax.sql.DataSource database2;

  public void someMethod(...) {
      java.sql.Connection con1;
      java.sql.Connection con2;
      java.sql.Statement stmt1;
      java.sql.Statement stmt2;
      // obtain con1 object and set it up for transactions
      con1 = database1.getConnection();
      stmt1 = con1.createStatement();
      // obtain con2 object and set it up for transactions
      con2 = database2.getConnection();
      stmt2 = con2.createStatement();
      //
      // Now do a transaction that involves con1 and con2.
      //
      // start the transaction
      ut.begin();
      // Do some updates to both con1 and con2. The container
      // automatically enlists con1 and con2 with the transaction. stmt1.executeQuery(...);
      stmt1.executeUpdate(...);
      stmt2.executeQuery(...);
      stmt2.executeUpdate(...);
      stmt1.executeUpdate(...);
      stmt2.executeUpdate(...);
      // commit the transaction
      ut.commit();
      // release connections
      stmt1.close();
      stmt2.close();
      con1.close();
      con2.close();
}
... }
What is wrong with this code? Well, first of all it does not compile. Quite honestly I wouldn't really care too much about that - an example is fine as long as the important point is still brought across. But in this case it does matter as we will see later on. So here is what the method should read like:
public void someMethod() throws SQLException, NotSupportedException,
  SystemException, SecurityException, IllegalStateException,
  RollbackException, HeuristicMixedException,
  HeuristicRollbackException {
In the original example any code concerning Exceptions has been left out. So what happens if an Exceptions is thrown? No close() is ever called and therefore none of the resources are ever cleaned up. Let's fix this:
@Stateless
@TransactionManagement(TransactionManagementType.BEAN)
public class MySessionBean {
  @Resource
  javax.transaction.UserTransaction ut;
  @Resource
  javax.sql.DataSource database1;
  @Resource
  javax.sql.DataSource database2;

  public void someMethod() throws SQLException, NotSupportedException,
      SystemException, SecurityException, IllegalStateException,
      RollbackException, HeuristicMixedException,
      HeuristicRollbackException {
    java.sql.Connection con1 = null;
    java.sql.Connection con2 = null;
    java.sql.Statement stmt1 = null;
    java.sql.Statement stmt2 = null;
    try {

      con1 = database1.getConnection();
      stmt1 = con1.createStatement();

      con2 = database2.getConnection();
      stmt2 = con2.createStatement();

      ut.begin();

      stmt1.executeUpdate("");
      stmt2.executeQuery("");
      stmt2.executeUpdate("");
      stmt1.executeUpdate("");
      stmt2.executeUpdate("");

      ut.commit();
    } finally {
      if (stmt2 != null)
        try {
          stmt2.close();
        } catch (SQLException ex) {
        }
      if (con2 != null)
        try {
          con2.close();
        } catch (SQLException ex) {
        }
      if (stmt1 != null)
        try {
          stmt1.close();
        } catch (SQLException ex) {
        }

      if (con1 != null)
        try {
          con1.close();
        } catch (SQLException ex) {
        }

    }
  }
}
This is the infamous try-catch-finally-try-catch block. The null checks actually avoid calling close() on objects that have not been created - as the creation might already have failed and and exception might have been thrown. As the close() operation might throw an exception this also needs to be handled. IMHO it is OK to swallow the exception - there is not really anything that can be done as the resources are already being closed. Using Spring's JdbcTemplate would have avoided the problem as the resource handling is done by the template then. I would strongly recommend it in code like this. It can be used independently from the other parts of Spring - e.g. Dependency Injection can still be done by Java EE. See http://static.springsource.org/spring/docs/3.1.x/spring-framework-reference/html/jdbc.html#jdbc-core for details. Now there is another thing that is still not OK - actually the most important point. The UserTransaction is never committed nor rolled back if an exception occurs. Let's fix this, too:
@Stateless
@TransactionManagement(TransactionManagementType.BEAN)
public class MySessionBean {
  @Resource
  javax.transaction.UserTransaction ut;
  @Resource
  javax.sql.DataSource database1;
  @Resource
  javax.sql.DataSource database2;

  public void someMethod() throws Exception {
    java.sql.Connection con1 = null;
    java.sql.Connection con2 = null;
    java.sql.Statement stmt1 = null;
    java.sql.Statement stmt2 = null;
    try {

      con1 = database1.getConnection();
      stmt1 = con1.createStatement();

      con2 = database2.getConnection();
      stmt2 = con2.createStatement();

      ut.begin();

      stmt1.executeUpdate("");
      stmt2.executeQuery("");
      stmt2.executeUpdate("");
      stmt1.executeUpdate("");
      stmt2.executeUpdate("");

    } catch (Exception ex) {
      ut.setRollbackOnly();
      throw ex;
    } finally {
      ut.commit();
      if (stmt2 != null)
        try {
          stmt2.close();
        } catch (SQLException ex) {
        }
      if (con2 != null)
        try {
          con2.close();
        } catch (SQLException ex) {
        }
      if (stmt1 != null)
        try {
          stmt1.close();
        } catch (SQLException ex) {
        }

      if (con1 != null)
        try {
          con1.close();
        } catch (SQLException ex) {
        }

    }
  }
}
So when a problem occurs the transaction will now be marked as rollback only and ultimately rolled back. If there is no exception it will be committed. I hope I made no further mistakes in the code - let me know otherwise. In Spring resource handling is done for you by the Templates and therefore I might be doing something wrong here. Using that approach would also have made the code a lot less complex. So why this blog post? Bad resource handling and transaction handling is far too common. I have done a lot of reviews and usually Enterprise Java applications fail to do a good job in this regard. This is dangerous, because Sadly the EJB spec seem to do resource handling constantly wrong. The EJB spec is not the only document that does resource and transaction handling wrong. Quite the contrary: I have a read a lot of tutorial and other documents that get this entirely wrong. However, IMHO at least the example in the spec for using the UserTransaction should show how to use it in a bullet proof manner. This document is read by many developers and other authors. Bad practices in such a document might end up in a lot of code and other publications.
 
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Leobschützer Strasse 22
13125 Berlin
E-Mail-Adresse: eberhard.wolff@gmail.com

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