Add singleton-aware DI with constructor injection

Todo.md

@@ -7,7 +7,7 @@ Parameter-agnostic methods
 
 
 - DI framework
-Autowiring
+Autowiring (done)
 Maybe explicit mapping/configuration
 
 - error handling

src/main/java/com/norwood/core/AnnotationProcessor.java

@@ -40,16 +40,12 @@ public void processAnnotations(List<Class<?>> classDefinitions, Router router) {
 
     private void inject(Field field) {
         try {
-            Class<?> fieldType = field.getType();
             Object owner = container().get(field.getDeclaringClass());
-            Object dependency = fieldType.getDeclaredConstructor().newInstance();
+            Object dependency = resolveDependency(field.getType());
 
             field.setAccessible(true);
             field.set(owner, dependency);
-
-            System.out.println(owner);
-            System.out.println(dependency);
-        } catch (InstantiationException  | 
+        } catch (InstantiationException  |
                 IllegalAccessException | IllegalArgumentException |
                 InvocationTargetException | NoSuchMethodException e)
         {
@@ -58,6 +54,47 @@ private void inject(Field field) {
         }
     }
 
+    private Object resolveDependency(Class<?> fieldType)
+            throws InstantiationException, IllegalAccessException,
+                   IllegalArgumentException, InvocationTargetException,
+                   NoSuchMethodException {
+        Object existing = container().get(fieldType);
+        if (existing != null && fieldType.isAnnotationPresent(Singleton.class)) {
+            return existing;
+        }
+
+        Object instance = instantiate(fieldType);
+
+        if (fieldType.isAnnotationPresent(Singleton.class)) {
+            container().set(fieldType, instance);
+        }
+
+        return instance;
+    }
+
+    private Object instantiate(Class<?> clazz)
+            throws InstantiationException, IllegalAccessException,
+                   IllegalArgumentException, InvocationTargetException,
+                   NoSuchMethodException {
+        // Prefer zero argument constructor
+        try {
+            var ctor = clazz.getDeclaredConstructor();
+            ctor.setAccessible(true);
+            return ctor.newInstance();
+        } catch (NoSuchMethodException e) {
+            // Fall back to first constructor and resolve parameters
+        }
+
+        var ctor = clazz.getDeclaredConstructors()[0];
+        ctor.setAccessible(true);
+        Class<?>[] paramTypes = ctor.getParameterTypes();
+        Object[] params = new Object[paramTypes.length];
+        for (int i = 0; i < paramTypes.length; i++) {
+            params[i] = resolveDependency(paramTypes[i]);
+        }
+        return ctor.newInstance(params);
+    }
+
     private void routePost(Post a, Router router, Method method) {
         String path = a.path();
         if (router.hasRouteWithPath(path)) {

src/main/java/com/norwood/core/KatanaContainer.java

@@ -17,8 +17,13 @@ public <T> T get(Class<T> beanClass) {
     @Override
     public <T> void set(Class<T> beanClass, T bean) {
         if (beans.get(beanClass) != null) {
+            // Ignore subsequent registrations for singletons
+            if (beanClass.isAnnotationPresent(Singleton.class)) {
+                return;
+            }
             throw new BeanAlreadyDefinedException("Bean already defined.");
         }
+
         beans.put(beanClass, bean);
     }
 

src/test/java/com/norwood/AutowireTest.java

@@ -0,0 +1,42 @@
+package com.norwood;
+
+import junit.framework.TestCase;
+
+import com.norwood.core.AnnotationProcessor;
+import com.norwood.core.KatanaContainer;
+import com.norwood.core.Singleton;
+import com.norwood.core.annotations.Inject;
+import com.norwood.routing.Router;
+
+public class AutowireTest extends TestCase {
+    @Singleton
+    public static class SingletonService {}
+
+    public static class NoDefaultConstructor {
+        final SingletonService service;
+        public NoDefaultConstructor(SingletonService service) {
+            this.service = service;
+        }
+    }
+
+    public static class ClientBean {
+        @Inject SingletonService singleton;
+        @Inject NoDefaultConstructor nodefault;
+    }
+
+    public void testAutowiring() {
+        KatanaContainer container = new KatanaContainer();
+        container.set(ClientBean.class, new ClientBean());
+        container.set(SingletonService.class, new SingletonService());
+
+        AnnotationProcessor processor = new AnnotationProcessor();
+        processor.processAnnotations(container.classDefinitions(), new Router());
+
+        ClientBean bean = container.get(ClientBean.class);
+        SingletonService svc = container.get(SingletonService.class);
+
+        assertSame(svc, bean.singleton);
+        assertNotNull(bean.nodefault);
+        assertSame(svc, bean.nodefault.service);
+    }
+}