在Lift2.0当中，AnyVarTrait是一个非常重要的trait,很多重要的像RequestVar,SessionVar都是继续该接口的,

首先看下大致接口代码

abstract class AnyVar[T, MyType <: AnyVar[T, MyType]](dflt: => T) extends AnyVarTrait[T, MyType] {self: MyType =>protected def calcDefaultValue: T = dflt
}/*** Abstract a request or a session scoped variable.*/
trait AnyVarTrait[T, MyType <: AnyVarTrait[T, MyType]] extends PSettableValueHolder[T] with HasCalcDefaultValue[T] {

AnyVarTrait继承了PSettableValueHolder,它的接口是:

trait ValueHolder {type ValueTypedef is: ValueTypedef get: ValueType
}trait SettableValueHolder extends ValueHolder {def set(in: ValueType): ValueType
}trait PValueHolder[T] extends ValueHolder {type ValueType = T// def manifest: Manifest[T]
}trait PSettableValueHolder[T] extends PValueHolder[T] with SettableValueHolder}
}

PSettableValueHolder主要是实现了类似于get,set的这样的方法，比较简单，写一个比较简单的实现PSettableValueHolder的例子:

class Exampletrait ValueHolder{type ValueTypedef is: ValueTypedef get: ValueType
}trait SettableValueHolder extends ValueHolder{def set(in: ValueType): ValueType
}trait PValueHolder[T] extends ValueHolder{type ValueType = T
}trait PSettableValueHolder[T] extends PValueHolder[T] with SettableValueHolderclass TestPSettableValueHolder[T] extends PSettableValueHolder[T]
{protected var testVal: T=_def set(in: T):T ={testVal=inin}def get:T={testVal}def is:T={testVal}
}object Test{def main(args: Array[String]){val test=new TestPSettableValueHolder[String]test.set("Test22")println(test.get)}
}

需要重载的方法:

protected lazy val name = VarConstants.varPrefix+getClass.getName+"_"+__nameSaltprotected def findFunc(name: String): Box[T]protected def setFunc(name: String, value: T): Unitprotected def clearFunc(name: String): Unitprotected def wasInitialized(name: String): Booleanprotected def calcDefaultValue: T/*** A non-side-effecting test if the value was initialized*/protected def testWasSet(name: String): Booleanprotected def __nameSalt = ""type CleanUpParam/*** Different Vars require different mechanisms for synchronization.  This method implements* the Var specific synchronization mechanism*/def doSync[F](f: => F): F

实现了PSettableValueHolder[T] trait的方法:

/*** The current value of the variable*/def is: T = doSync {findFunc(name) match {case Full(v) => vcase _ => val ret = calcDefaultValuetestInitializedapply(ret)// Use findFunc so that we clear the "unread" flagfindFunc(name) match {case Full(v) => vcase _ => ret}}}private def testInitialized: Unit = doSync {if (!wasInitialized(name)) {registerCleanupFunc(_onShutdown _)}}/*** Shadow of the 'is' method*/def get: T = is/*** Shadow of the apply method*/def set(what: T): T = apply(what)

该trait的apply实现:

/*** Set the session variable** @param what -- the value to set the session variable to*/def apply(what: T): T = {testInitializedsetFunc(name, what)what}

最后一个该trait非常重要的方法 :在该var的生命周期内去修改它的Value:

  /*** Change the value of the Var for the lifespan of the function*/def doWith[F](newVal: T)(f: => F): F = {val old = findFunc(name)setFunc(name, newVal)try {f} finally {old match {case Full(t) => setFunc(name, t)case _ => clearFunc(name)}}}

再来看下该trait的具体实现:SessionVar和RequestVar

SessionVar,看下他的几个主要实现AnyVarTrait的实现:

 override protected def findFunc(name: String): Box[T] = S.session match {case Full(s) => s.get(name)case _ =>if (showWarningWhenAccessedOutOfSessionScope_?)logger.warn("Getting a SessionVar "+name+" outside session scope") // added warning per issue 188Empty}override protected def setFunc(name: String, value: T): Unit = S.session match {case Full(s) => s.set(name, value)case _ =>if (showWarningWhenAccessedOutOfSessionScope_?)logger.warn("Setting a SessionVar "+name+" to "+value+" outside session scope") // added warning per issue 188}/*** Different Vars require different mechanisms for synchronization.  This method implements* the Var specific synchronization mechanism*/def doSync[F](f: => F): F = S.session match {case Full(s) =>// lock the session while the Var-specific lock object is found/createdval lockName = name + VarConstants.lockSuffixval lockObj = s.synchronized {s.get[AnyRef](lockName) match {case Full(lock) => lockcase _ => val lock = new AnyRefs.set(lockName, lock)lock}}// execute the query in the scope of the lock objlockObj.synchronized {f}case _ => f}

RequestVar的实现:

override protected def findFunc(name: String): Box[T] = RequestVarHandler.get(name)override protected def setFunc(name: String, value: T): Unit = RequestVarHandler.set(name, this, value)override protected def clearFunc(name: String): Unit = RequestVarHandler.clear(name)override protected def wasInitialized(name: String): Boolean = {val bn = name + VarConstants.initedSuffixval old: Boolean = RequestVarHandler.get(bn) openOr falseRequestVarHandler.set(bn, this, true)old}/*** Different Vars require different mechanisms for synchronization.  This method implements* the Var specific synchronization mechanism*/def doSync[F](f: => F): F = f // no sync necessary for RequestVars... always on the same thread