Add answer to exercise 8 chapter 3

src/main/scala/org/learningconcurrency/exercises/ch3/ex8.scala

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+package org.learningconcurrency
+package exercises
+package ch3
+
+import java.io.File
+import java.io.FileInputStream
+import java.io.FileOutputStream
+import java.io.ObjectInputStream
+import java.io.ObjectOutputStream
+import java.util.regex.Pattern
+
+import scala.sys.process._
+
+/**
+ * Implement a method spawn that, given a block of Scala code, starts a new JVM process and runs the specified block in the new process:
+ * def spawn[T](block: =>T): T = ???
+ * Once the block returns a value, the spawn method should return the value from the child process.
+ * If the block throws an exception, the spawn method should throw the same exception.
+ */
+object Ex8 extends App {
+
+  // This method's preconditions are the following:
+  //   - In case of executing in sbt, set `fork` setting to `true` (set fork := true ).
+  //
+  // If passed block which contains `System.exit`, this method throws `SecurityException`.
+  def spawn[T](block: => T): T = {
+    val className = Ex8_EvaluationApp.getClass().getName().split((Pattern.quote("$")))(0)
+    val tmp = File.createTempFile("concurrent-programming-in-scala", null)
+    tmp.deleteOnExit()
+
+    val out = new ObjectOutputStream(new FileOutputStream(tmp))
+    try {
+      out.writeObject(() => block)
+    } finally {
+      out.close()
+    }
+
+    val ret = Process(s"java -cp ${System.getProperty("java.class.path")} $className ${tmp.getCanonicalPath}").!
+    if (ret != 0)
+      throw new RuntimeException("fails to evaluate block in a new JVM process")
+
+    val in = new ObjectInputStream(new FileInputStream(tmp))
+    try {
+      in.readObject() match {
+        case e: Throwable => throw e
+        case x => x.asInstanceOf[T]
+      }
+    } finally {
+      in.close()
+      tmp.delete()
+    }
+  }
+
+  val s1 = spawn({
+    1 + 1
+  })
+  assert(s1 == 2)
+
+  try {
+    spawn({
+      "test".toInt
+    })
+  } catch {
+    case e: NumberFormatException =>
+    case _: Throwable => assert(false)
+  }
+
+  try {
+    spawn({
+      System.exit(0)
+    })
+  } catch {
+    case e: SecurityException =>
+    case _: Throwable => assert(false)
+  }
+}

src/main/scala/org/learningconcurrency/exercises/ch3/ex8_evaluationapp.scala

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+package org.learningconcurrency
+package exercises
+package ch3
+
+import java.io.FileInputStream
+import java.io.FileOutputStream
+import java.io.ObjectInputStream
+import java.io.ObjectOutputStream
+import java.security.Permission
+
+// This application receives the file path in which the serialized `Function0` object has been written.
+// Then, it reads and evaluates serialized `Function0` object, finally overwrites its result to the same file.
+object Ex8_EvaluationApp extends App {
+  System.setSecurityManager(new SecurityManager() {
+    // allows access to file.
+    override def checkPermission(perm: Permission): Unit = {}
+    // detects `System.exit` in order not to be halted by the caller.
+    override def checkExit(status: Int): Unit = {
+      throw new SecurityException("not allowed to pass a block which contains System.exit(int) !")
+    }
+  });
+
+  val path = args(0)
+
+  val in = new ObjectInputStream(new FileInputStream(path))
+  try {
+    val f0 = in.readObject().asInstanceOf[Function0[Any]]
+    in.close()
+
+    val out = new ObjectOutputStream(new FileOutputStream(path))
+    try {
+      out.writeObject(f0())
+    } catch {
+      case e: Throwable => out.writeObject(e)
+    } finally {
+      out.close()
+    }
+  } finally {
+    in.close()
+  }
+}