@specialized 主要用在Scala的范型上，使得Scala编译器能够在编译的时候，为某些primative类型提供一个更加高效的实现。这种高效源于避免了对指定primative类型的boxing和unboxing(int -> Int -> int)[1]。根据一个相关研究[2]，boxing和unboxing还是很花时间的。比如
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+ | class My [A] {
+ def iden(x: A): A = x
+}
+val a = new My[Int]
+for (i <- 1 to 1000000) a.iden(i)
+
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需要花40纳秒。改为：
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+ | class My [@specialized(Int) A] {
+ def iden(x: A): A = x
+}
+val a = new My[Int]
+for (i <- 1 to 1000000) a.iden(i)
+
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就只需要17纳秒。
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下面看一个简单的用@specialized的例子：
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+ | class Example[@specialized(Int) T](value: T) {
+ def get(): T = value
+}
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用命令scalac -print test.scala编译后变为：
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+ | [[syntax trees at end of cleanup]]// Scala source: test.scala
+ package <empty> {
+ class Example extends java.lang.Object with ScalaObject {
+ <paramaccessor> protected[this] val value: java.lang.Object = _;
+ def get(): java.lang.Object = Example.this.value;
+ <specialized> def get$mcI$sp(): Int = scala.Int.unbox(Example.this.get());
+ def this(value: java.lang.Object): Example = {
+ Example.this.value = value;
+ Example.super.this();
+ ()
+ }
+ };
+ <specialized> class Example$mcI$sp extends Example {
+ <paramaccessor> <specialized> protected[this] val value$mcI$sp: Int = _;
+ override <specialized> def get(): Int = Example$mcI$sp.this.get$mcI$sp();
+ override <specialized> def get$mcI$sp(): Int = Example$mcI$sp.this.value$mcI$sp;
+ override <bridge> <specialized> def get(): java.lang.Object = scala.Int.box(Example$mcI$sp.this.get());
+ <specialized> def this(value$mcI$sp: Int): Example$mcI$sp = {
+ Example$mcI$sp.this.value$mcI$sp = value$mcI$sp;
+ Example$mcI$sp.super.this(scala.Int.box(value$mcI$sp));
+ ()
+ }
+ }
+ }
+
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可见有两个类被生成了。如果把源文件变为：
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+ | class Example[@specialized(Int, Long) T](value: T) {
+ def get(): T = value
+}
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则有第三个类被生成：
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+ | [[syntax trees at end of cleanup]]// Scala source: test.scala
+ package <empty> {
+ ...
+ ...
+ <specialized> class Example$mcJ$sp extends Example {
+ <paramaccessor> <specialized> protected[this] val value$mcJ$sp: Long = _;
+ override <specialized> def get(): Long = Example$mcJ$sp.this.get$mcJ$sp();
+ override <specialized> def get$mcJ$sp(): Long = Example$mcJ$sp.this.value$mcJ$sp;
+ override <bridge> <specialized> def get(): java.lang.Object = scala.Long.box(Example$mcJ$sp.this.get());
+ <specialized> def this(value$mcJ$sp: Long): Example$mcJ$sp = {
+ Example$mcJ$sp.this.value$mcJ$sp = value$mcJ$sp;
+ Example$mcJ$sp.super.this(scala.Long.box(value$mcJ$sp));
+ ()
+ }
+ }
+ }
+
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+理想情况是很多范型都对所有primative类型做specialized，不过这样编译后的代码就会成倍的增加了。Scala 2.8.1中只有下面的类有用到@specialized[3]:
+Function0, Function1, Function2, Tuple1, Tuple2, Product1, Product2, AbstractFunction0, AbstractFunction1, AbstractFunction2.
+