java - 在 Java 中评估 Scala 脚本时脚本引擎出错

Question

我有一个应用程序可以执行传递给它的 Scala 脚本。以下是我如何尝试执行 Scala 代码的示例。

String script = "println(\"Hello World!\")";
ScriptEngine scriptEngine = new ScriptEngineManager().getEngineByName("scala");
scriptEngine.eval(script);

我收到以下错误。

[init] error: error while loading Object, Missing dependency 'object scala in compiler mirror', required by /Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre/lib/rt.jar(java/lang/Object.class)

Failed to initialize compiler: object scala in compiler mirror not found.
** Note that as of 2.8 scala does not assume use of the java classpath.
** For the old behavior pass -usejavacp to scala, or if using a Settings
** object programmatically, settings.usejavacp.value = true.

我在 Mac 上使用 Java 7 (jdk1.7.0_80.jdk)。我在 spring-boot 应用程序中运行它。

使用了 Scala maven 依赖项。

<dependency>
    <groupId>org.apache.clerezza.scala</groupId>
    <artifactId>script-engine</artifactId>
    <version>1.0.0</version>
</dependency>

为什么会出现此错误？

Answer 1

我们需要像这样导入和初始化脚本引擎:

import javax.script.*;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

public class TestScript {

    public static void main(String... args) throws Exception {
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("scala");

...

使用 ScriptEngineManager().getEngineByName("scala") 可以查找匹配的脚本引擎。这只是为我们提供了引擎，而不是真正执行脚本所需的库和 Hook 。

关于 Scala 加载 JVM 所需的标准类的方式，有一点需要注意。在执行 scala 的情况下，标准 Scala 库被放置在 JVM 的类路径中。这里不会是这种情况。你可以阅读this供引用。尝试运行示例类会导致以下异常:

reflect.jar:. TestScript
[init] error: error while loading Object, Missing dependency 'object scala in compiler mirror', required by /Library/Java/JavaVirtualMachines/jdk1.8.0_65.jdk/Contents/Home/jre/lib/rt.jar(java/lang/Object.class)

Failed to initialize compiler: object scala in compiler mirror not found.
** Note that as of 2.8 scala does not assume use of the java classpath.
** For the old behavior pass -usejavacp to scala, or if using a Settings
** object programmatically, settings.usejavacp.value = true.
Exception in thread "main" scala.reflect.internal.MissingRequirementError: object scala in compiler mirror not found.

我们可以通过使用 scala java 工具帮助我们将标准 Scala 库加载到 JVM 类路径中来解决这个问题。这是使我们的 Scala 脚本工作的一种方法:

import javax.script.*;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

import scala.tools.nsc.interpreter.IMain;
import scala.tools.nsc.settings.MutableSettings.BooleanSetting;

public class TestScript {

    public static void main(String... args) throws Exception {
          ....
          ((BooleanSetting)(((IMain)engine).settings()
                               .usejavacp())).value_$eq(true);

....

另一种是使用以下选项简单地执行代码:

$ java -Dscala.usejavacp=true ...

完整的脚本是这样的:

import javax.script.*;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

import scala.tools.nsc.interpreter.IMain;
import scala.tools.nsc.settings.MutableSettings.BooleanSetting;


public class TestScript {

    public static void main(String... args) throws Exception {
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("scala");

        ((BooleanSetting)(((IMain)engine)
               .settings().usejavacp()))
                    .value_$eq(true);

        String testScript = "var a:Int =  10";
        engine.eval(testScript);

        String testScript2 = "println(a)";
        engine.eval(testScript2);

        String testScript3 = "println(a+5)";
        engine.eval(testScript3);
    }
}

编译和运行它非常简单，因为我们需要类路径中的 Scala 库。

编译:

$ javac -cp /Users//hkropp/bin/scala-2.11.7/lib/scala-library.jar:\
/Users//hkropp/bin/scala-2.11.7/lib/scala-compiler.jar:\
/Users//hkropp/bin/scala-2.11.7/lib/scala-reflect.jar \
TestScript.java

运行:

$ java -Dscala.usejavacp=true \-cp /Users//hkropp/bin/scala-2.11.7/lib/scala-library.jar:\
/Users//hkropp/bin/scala-2.11.7/lib/scala-compiler.jar:\
/Users//hkropp/bin/scala-2.11.7/lib/scala-reflect.jar:. \
TestScript
10
15