从字符串生成类并在Scala 2.10中实例化它 [英] Generating a class from string and instantiating it in Scala 2.10

问题描述

在Scala 2.10中，如何稍后通过字符串(可能使用Toolbox api)从字符串生成一个类，以使用Scala的反射进行实例化?

In Scala 2.10 how do I generate a class from string (probably, using the Toolbox api) later to be instantiated with Scala's reflection?

推荐答案

W.r.t编译工具箱只能运行expressions =返回值，而不能生成带有编译结果的类或文件/字节数组.

W.r.t compilation toolboxes can only run expressions = return values, but not resulting classes or files/byte arrays with compilation results.

但是仍然可以实现您想要的，因为在Scala中，使用隐式值从类型级别转换到值级别非常容易:

However it's still possible to achieve what you want, since in Scala it's so easy to go from type level to value level using implicit values:

修改.在2.10.0-RC1中，ToolBox的某些方法已重命名. parseExpr现在只是parse，而runExpr现在称为eval.

Edit. In 2.10.0-RC1 some methods of ToolBox have been renamed. parseExpr is now just parse, and runExpr is now called eval.

scala> import scala.reflect.runtime._ // requires scala-reflect.jar
                                      // in REPL it's implicitly added 
                                      // to the classpath
                                      // but in your programs
                                      // you need to do this on your own
import scala.reflect.runtime

scala> val cm = universe.runtimeMirror(getClass.getClassLoader)
cm @ 41d0fe80: reflect.runtime.universe.Mirror = JavaMirror with scala.tools.nsc.interpreter.IMain$TranslatingClassLoader...

scala> import scala.tools.reflect.ToolBox // requires scala-compiler.jar
                                          // in REPL it's implicitly added 
                                          // to the classpath
                                          // but in your programs
                                          // you need to do this on your own
import scala.tools.reflect.ToolBox

scala> val tb = cm.mkToolBox()
tb: scala.tools.reflect.ToolBox[reflect.runtime.universe.type] = scala.tools.reflect.ToolBoxFactory$ToolBoxImpl@3a962da5

scala> tb.runExpr(tb.parseExpr("class C; scala.reflect.classTag[C].runtimeClass"))
res2: Any = class __wrapper$1$f9d572ca0d884bca9333e251c64e980d$C$1

更新#1.如果不需要java.lang.Class，而只需要实例化已编译的类，则可以在提交给runExpr的字符串中直接编写new C.

Update #1. If you don't need a java.lang.Class and just need to instantiate the compiled class, you can write new C directly in the string submitted to runExpr.

更新#2. runExpr也可以使用从变量名到运行时值的自定义映射.例如:

Update #2. It is also possible to have runExpr use custom mapping from variable names to runtime values. For example:

scala> val build = scala.reflect.runtime.universe.build
build: reflect.runtime.universe.BuildApi = scala.reflect.internal.BuildUtils$BuildImpl@50d5afff

scala> val x = build.setTypeSignature(build.newFreeTerm("x", 2), typeOf[Int])
x: reflect.runtime.universe.FreeTermSymbol = free term x

scala> tb.runExpr(Apply(Select(Ident(x), newTermName("$plus")), List(Literal(Constant(2)))))
res0: Any = 4

在此示例中，我创建一个值为2的自由项(该值不必是原始值，它可以是您的自定义对象)，并将标识符绑定到该值.然后，该值将按原样在由工具箱编译和运行的代码中使用.

In this example I create a free term that has a value of 2 (the value doesn't have to be a primitive - it can be your custom object) and bind an identifier to it. This value is then used as-is in the code that is compiled and run by a toolbox.

该示例使用手动AST汇编，但是可以编写一个函数来解析字符串，找出未绑定的标识符，在某些映射中查找它们的值，然后创建相应的自由术语.但是在Scala 2.10.0中没有这样的功能.

The example uses manual AST assembly, but it's possible to write a function that parses a string, finds out unbound identifiers, looks up values for them in some mapping and then creates corresponding free terms. There's no such function in Scala 2.10.0 though.

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