spring expression 学习（一）

Expression interface:
An expression capable of evaluating itself against context objects.
Encapsulates the details of previously parsed expression string.
Provides an common abstraction for expression evaluation independent of
any language like OGNL or the Unified EL.

EvaluationContext interface:
Expression was executed in an evaluation context.It is in this context references
are resolved when encountered during expression evaluation.

ExpressionParser interface:
Parses expression strings into compiled expressions that can be evaluated.
Supports parsing templates as well as standard expression strings.

ConstructorExecutor interface:
Executors are built by resolvers and can be cached by the infrastructure to repeat an operation quickly without going
back to the resolvers.
For example,the particular constructor to run on a class may be discovered by the reflection constructor resolver ,it
will build an constructorexecutor that executes that constructor and the constructorexecutor can be reused without
needing to go back to the resolver to discover the constructor again.
They can become stale,and in that case should throw an accessexception,this will cause the infrastructure to go back
to the resolvers to ask a new one.

ConstructorResolver interface:
A constrctor resolver attemps locate a constructor and returns a constructorexecutor that can be
used to invoke the constructor.The ConstructorExecutor can be cached but if it 'goes stale' the
resolvers will be called again.