Explanation/Reference:
M1 has no transaction.
As M2 has Required attribute it will run within a new transaction.
Note:
* Required Attribute
If the client is running within a transaction and invokes the enterprise bean's method, the method executes within the client's transaction. If the client is not associated with a transaction, the container starts a new transaction before running the method.
The Requiredattribute is the implicit transaction attribute for all enterprise bean methods running with container-managed transaction demarcation. You typically do not set the Requiredattribute unless you need to override another transaction attribute. Because transaction attributes are declarative, you can easily change them later.
Transaction Attributes
A transaction attribute controls the scope of a transaction. Figure below illustrates why controlling the scope is important. In the diagram, method-A begins a transaction and then invokes method-B of Bean-2.
When method-B executes, does it run within the scope of the transaction started by method-A, or does it execute with a new transaction? The answer depends on the transaction attribute of method-B.
Reference: The Java EE 6 Tutorial, Container-Managed Transactions