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org.apache.derby.iapi.services.context
Interface Context

All Known Implementing Classes:: ContextImpl

public interface Context

Contexts are created and used to manage the execution environment. They provide a convenient location for storing globals organized by the module using the globals.

A basic context implementation is provided as an abstract class; this implementation satisfies the interface and should in general be used as the supertype of all context types. Otherwise, context classes must satisfy the semantics of the interface through their own distinct implementations.

Contexts assist in cleanup when errors are caught in the outer block.

Use of context cleanup is preferred over using try/catch blocks throughout the code.

Use of context pushing and popping is preferred over using many instance or local variables, even when try/catch is present. when the instance or local variables would be holding resources.

Usually Context's have a reference based equality, ie. they do not provide an implementation of equals(). Contexts may implement a value based equality but this usually means there is only one instance of the Context on the stack, This is because the popContext(Context) will remove the most recently pushed Context that matches via the equals method, not by a reference check. Implementing equals is useful for Contexts used in notifyAllThreads() that is not aimed at a single thread.

Method Summary

void cleanupOnError(java.lang.Throwable error)
          Contexts will be passed errors that are caught by the outer system when they are serious enough to require corrective action.

ContextManager getContextManager()
          Returns the context manager that has stored this context in its stack.

java.lang.String getIdName()
          Returns the current id name associated with this context.

boolean isLastHandler(int severity)
          Return whether or not this context is the "last" handler for a the specified severity level.

void popMe()
          Pop myself of the context stack.

void pushMe()
          Push myself onto my context stack.

Method Detail

getContextManager

public ContextManager getContextManager()

Returns the context manager that has stored this context in its stack.

getIdName

public java.lang.String getIdName()

Returns the current id name associated with this context. Contexts are placed into stacks by id, in a context manager. Null if the context is not assigned to an id. Contexts known by context managers are always assigned to an id.

A default Id name should be defined in each specific context interface as a static final field with the name CONTEXT_ID. For example, see org.apache.derby.iapi.sql.compile.CompilerContext.CONTEXT_ID.

cleanupOnError

public void cleanupOnError(java.lang.Throwable error)
                    throws org.apache.derby.iapi.error.StandardException

Contexts will be passed errors that are caught by the outer system when they are serious enough to require corrective action. They will be told what the error is, so that they can react appropriately. Most of the time, the contexts will react by either doing nothing or by removing themselves from the context manager. If there are no other references to the context, removing itself from the manager equates to freeing it.

Contexts must release all their resources before removing themselves from their context manager.

The context manager will "unwind" the contexts during cleanup in the reverse order they were placed on its global stack.

If error is an instance of StandardException then an implementation of this method may throw a new exception if and only if the new exception is an instance of StandardException that is more severe than the original error or the new exception is a not an instance of StandardException (e.g java.lang.NullPointerException).

pushMe

public void pushMe()

Push myself onto my context stack.

popMe

public void popMe()

Pop myself of the context stack.

isLastHandler

public boolean isLastHandler(int severity)

Return whether or not this context is the "last" handler for a the specified severity level. Previously, the context manager would march through all of the contexts in cleanupOnError() and call each of their cleanupOnError() methods. That did not work with server side JDBC, especially for a StatementException, because outer contexts could get cleaned up incorrectly. This functionality is specific to the Language system. Any non-language system contexts should return ExceptionSeverity.NOT_APPLICABLE_SEVERITY. NOTE: Both the LanguageConnectionContext and the JDBC Connection Context are interested in session level errors because they both have clean up to do. This method allows both of them to return false so that all such handlers under them can do their clean up.