June 2018 Update

Task/Runtime-evaluation/Java/runtime-evaluation.java

@@ -0,0 +1,86 @@
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.lang.reflect.InvocationTargetException;
+import java.net.URI;
+import java.util.HashMap;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import javax.tools.FileObject;
+import javax.tools.ForwardingJavaFileManager;
+import javax.tools.JavaCompiler;
+import javax.tools.JavaFileObject;
+import javax.tools.SimpleJavaFileObject;
+import javax.tools.StandardJavaFileManager;
+import javax.tools.StandardLocation;
+import javax.tools.ToolProvider;
+
+public class Evaluator{
+    public static void main(String[] args){
+        new Evaluator().eval(
+            "SayHello",
+            "public class SayHello{public void speak(){System.out.println(\"Hello world\");}}",
+            "speak"
+        );
+    }
+
+    void eval(String className, String classCode, String methodName){
+        Map<String, ByteArrayOutputStream> classCache = new HashMap<>();
+        JavaCompiler                       compiler   = ToolProvider.getSystemJavaCompiler();
+
+        if ( null == compiler )
+            throw new RuntimeException("Could not get a compiler.");
+
+        StandardJavaFileManager                            sfm  = compiler.getStandardFileManager(null, null, null);
+        ForwardingJavaFileManager<StandardJavaFileManager> fjfm = new ForwardingJavaFileManager<StandardJavaFileManager>(sfm){
+            @Override
+            public JavaFileObject getJavaFileForOutput(Location location, String className, JavaFileObject.Kind kind, FileObject sibling)
+                    throws IOException{
+                if (StandardLocation.CLASS_OUTPUT == location && JavaFileObject.Kind.CLASS == kind)
+                    return new SimpleJavaFileObject(URI.create("mem:///" + className + ".class"), JavaFileObject.Kind.CLASS){
+                        @Override
+                        public OutputStream openOutputStream()
+                                throws IOException{
+                            ByteArrayOutputStream baos = new ByteArrayOutputStream();
+                            classCache.put(className, baos);
+                            return baos;
+                        }
+                    };
+                else
+                    throw new IllegalArgumentException("Unexpected output file requested: " + location + ", " + className + ", " + kind);
+            }
+        };
+        List<JavaFileObject> files = new LinkedList<JavaFileObject>(){{
+            add(
+                new SimpleJavaFileObject(URI.create("string:///" + className + ".java"), JavaFileObject.Kind.SOURCE){
+                    @Override
+                    public CharSequence getCharContent(boolean ignoreEncodingErrors){
+                        return classCode;
+                    }
+                }
+            );
+        }};
+
+        // Now we can compile!
+        compiler.getTask(null, fjfm, null, null, null, files).call();
+
+        try{
+            Class<?> clarse = new ClassLoader(){
+                @Override
+                public Class<?> findClass(String name){
+                    if (! name.startsWith(className))
+                        throw new IllegalArgumentException("This class loader is for " + className + " - could not handle \"" + name + '"');
+                    byte[] bytes = classCache.get(name).toByteArray();
+                    return defineClass(name, bytes, 0, bytes.length);
+                }
+            }.loadClass(className);
+
+            // Invoke a method on the thing we compiled
+            clarse.getMethod(methodName).invoke(clarse.newInstance());
+
+        }catch(ClassNotFoundException | InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException x){
+            throw new RuntimeException("Run failed: " + x, x);
+        }
+    }
+}