Collection of all ContextManagers that are open in the complete Derby system. A ContextManager is added when it is created with newContextManager and removed when the session is closed.

See Also

newContextManager(), SystemContext#cleanupOnError(Throwable)

Maintains a list of all the contexts that this thread has created and/or used. The object stored in the thread local varys according how this thread has been used and will be one of:

• null - the thread no affiliation with a context manager.
• ContextManager - the current thread has used or is using this context manager. If ContextManager.activeThread equals the current thread then the thread is currently active with the ContextManager. In this case ContextManager.activeCount will be greater than zero and represent the level of nested setCurrentContextmanager calls. If ContextManager.activeThread is null then no other thread is using the Contextmanager, if ContextManager.activeThread is not-null and not equal to the current thread then some other thread is using the context. It is assumed that only a single thread can be using a ContextManager at any time and this is enforced by synchronization outside the ContextManager. E.g for JDBC connections, synchronization at the JDBC level.
• ContextManagerStack containing ContextManagers - the current thread is actively using multiple different ContextManagers, with nesting. All ContextManagers in the stack will have activeThread set to the current thread, and their activeCount set to -1. This is because nesting is solely represented by the stack, with the current context manager on top of the stack. This supports multiple levels of nesting across two stacks, e.g. C1->C2->C2->C1->C2.

There are two cases we are trying to optimise.

• Typical JDBC client program where there a Connection is always executed using a single thread. In this case this variable will contain the Connection's context manager
• Typical application server pooled connection where a single thread may use a connection from a pool for the lifetime of the request. In this case this variable will contain a WeakReference.

        threadContextList.get() == cm
        // while in JDBC engine code
        cm.activeThread == Thread.currentThread();
        cm.activeCount = 1;
        

        // thread executing request
         threadContextList.get() == cm
        // while in JDBC engine code
        cm.activeThread == Thread.currentThread();
        cm.activeCount = 1;

        // other threads that have recently executed
        // the same connection can have
        threadContextList.get() == cm
        cm.activeThread != Thread.currentThread();
       

        threadContextList.get() == cm
        // Within server-side JDBC code in a
        // function called from another function/procedure
        // called from an applications's statement
        // (three levels of nesting)
        cm.activeThread == Thread.currentThread();
        cm.activeCount = 3;         
        

        threadContextList.get() == stack {cm2,cm,cm}
        cm.activeThread == Thread.currentThread();
        cm.activeCount = -1; // nesting in stack
        cm2.activeThread == Thread.currentThread();
        cm2.activeCount = -1; // nesting in stack
        

        threadContextList.get() == stack {cm3,cm2,cm,cm2,cm,cm}
        cm.activeThread == Thread.currentThread();
        cm.activeCount = -1; // nesting in stack
        cm2.activeThread == Thread.currentThread();
        cm2.activeCount = -1; // nesting in stack
        cm3.activeThread == Thread.currentThread();
        cm3.activeCount = -1; // nesting in stack