Add ability to customize the injector with custom bootstrap modules

src/main/java/com/microsoft/dhalion/HealthManager.java

@@ -101,15 +101,27 @@ protected void configure() {
 
     cb.loadConfig(conf).loadPolicyConf();
     config = cb.build();
+    injector = injector.createChildInjector(new AbstractModule() {
+      @Override
+      protected void configure() {
+        bind(Config.class).toInstance(config);
+      }
+    });
+
+    //Register additional bindings
+    String bootstrapModuleClassName = (String) conf.get(Key.BOOTSTRAP_MODULE_CLASS.value());
+    Class<IBootstrapModule> bootstrapModuleClass = loadClass(bootstrapModuleClassName);
+    IBootstrapModule bootstrapModule = injector.getInstance(bootstrapModuleClass);
+
+    injector = injector.createChildInjector(bootstrapModule.get());
 
     //Read the MetricsProvider class
     String metricsProviderClass = (String) conf.get(Key.METRICS_PROVIDER_CLASS.value());
-    Class<MetricsProvider> mpClass
-        = (Class<MetricsProvider>) this.getClass().getClassLoader().loadClass(metricsProviderClass);
+    Class<MetricsProvider> mpClass = loadClass(metricsProviderClass);
+
     injector = injector.createChildInjector(new AbstractModule() {
       @Override
       protected void configure() {
-        bind(Config.class).toInstance(config);
         bind(mpClass).in(Singleton.class);
       }
     });
@@ -131,8 +143,7 @@ private void initializePolicies() throws ClassNotFoundException {
     for (PolicyConfig policyConf : config.policies()) {
       String policyClassName = policyConf.policyClass();
       LOG.info(String.format("Initializing %s with class %s", policyConf.id(), policyClassName));
-      Class<IHealthPolicy> policyClass
-          = (Class<IHealthPolicy>) this.getClass().getClassLoader().loadClass(policyClassName);
+      Class<IHealthPolicy> policyClass = loadClass(policyClassName);
 
       AbstractModule module = constructPolicySpecificModule(policyConf);
       IHealthPolicy policy = injector.createChildInjector(module).getInstance(policyClass);
@@ -141,6 +152,11 @@ private void initializePolicies() throws ClassNotFoundException {
     }
   }
 
+  @SuppressWarnings("unchecked")
+  private <T> Class<T> loadClass(String className) throws ClassNotFoundException {
+      return (Class<T>) this.getClass().getClassLoader().loadClass(className);
+  }
+
   private AbstractModule constructPolicySpecificModule(final PolicyConfig policyConfig) {
     return new AbstractModule() {
       @Override

src/main/java/com/microsoft/dhalion/IBootstrapModule.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) Microsoft Corporation. All rights reserved.
+ *
+ * This program is made available under the terms of the MIT License.
+ * See the LICENSE file in the project root for more information.
+ */
+package com.microsoft.dhalion;
+
+import com.google.inject.AbstractModule;
+import com.google.inject.Module;
+
+/**
+ * {@link IBootstrapModule} should be used to customize the injector with additional bindings required for injecting
+ * the metrics provider and the health policies.
+ */
+public interface IBootstrapModule {
+
+  /**
+   * Dhalion will create a new injector that will inherit all its state from the existing injector and add this module
+   * (and binding definitions) to it.
+   * 
+   * @return {@link Module} with additional binding definitions
+   */
+  default Module get() {
+    return new AbstractModule() {
+      @Override
+      protected void configure() {
+      }
+    };
+  }
+
+  public static class DefaultModule implements IBootstrapModule{}
+}

src/main/java/com/microsoft/dhalion/conf/Key.java

@@ -15,12 +15,12 @@ public enum Key {
   POLICY_CONF_SENSOR_DURATION_SUFFIX(".duration"),
   CONF_COMPONENT_NAMES("component.names"),
 
-
+  BOOTSTRAP_MODULE_CLASS("bootstrap.module.class", "com.microsoft.dhalion.IBootstrapModule$DefaultModule"),
   METRICS_PROVIDER_CLASS("metrics.provider.class");
 
   private final String value;
   private final Object defaultValue;
-
+  
   Key(String value) {
     this(value, null);
   }