FIX java.lang.NullPointerException when no java compiler available (fixes #107)

Didn't manage to actually test it fixes the problem, since I didn't reproduce it.

At the minimum we will get a better error message, but we should even auto disable
 auto compile in that case.

Author: xhanin

restx-classloader/src/main/java/restx/classloader/CompilationManager.java

@@ -119,6 +119,11 @@ public CompilationManager(EventBus eventBus, Iterable<Path> sourceRoots, Path de
         this.settings = settings;
 
         javaCompiler = ToolProvider.getSystemJavaCompiler();
+        if (javaCompiler == null) {
+            throw new IllegalStateException(
+                    "trying to setup a compilation manager while no system compiler is available." +
+                            " This should be prevented by checking the system java compiler first.");
+        }
         fileManager = javaCompiler.getStandardFileManager(new DiagnosticCollector<JavaFileObject>(), Locale.ENGLISH, Charsets.UTF_8);
         try {
             if (!destination.toFile().exists()) {

restx-core/src/main/java/restx/Apps.java

@@ -43,6 +43,12 @@ public Apps(AppSettings settings) {
     }
 
     public CompilationManager newAppCompilationManager(EventBus eventBus, CompilationSettings compilationSettings) {
+        if (!hasSystemJavaCompiler()) {
+            throw new IllegalStateException(
+                    "trying to setup a compilation manager while no system compiler is available. " +
+                            "This should be prevented by checking the system java compiler first. " +
+                            "Use hasSystemJavaCompiler() to check that before calling this method.");
+        }
         if (hasJavadocTools()) {
             eventBus.register(new Object() {
                 @Subscribe
@@ -144,12 +150,22 @@ public void run() {
         return process;
     }
 
-    private boolean hasJavadocTools() {
+    public static boolean hasJavadocTools() {
         try {
             Class.forName("com.sun.tools.javadoc.Main");
             return true;
         } catch (Exception e) {
             return false;
         }
     }
+
+    public static boolean hasSystemJavaCompiler() {
+        try {
+            Class<?> tp = Class.forName("javax.tools.ToolProvider");
+            return tp.getMethod("getSystemJavaCompiler").invoke(null) != null;
+        } catch (Exception e) {
+            return false;
+        }
+    }
+
 }

restx-core/src/main/java/restx/RestxMainRouterFactory.java

@@ -1,6 +1,11 @@
 package restx;
 
-import com.google.common.base.*;
+import com.google.common.base.Optional;
+import com.google.common.base.Predicate;
+import com.google.common.base.Stopwatch;
+import com.google.common.base.Supplier;
+import com.google.common.base.Suppliers;
+import com.google.common.base.Throwables;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.eventbus.EventBus;
 import com.google.common.eventbus.Subscribe;
@@ -13,7 +18,11 @@
 import restx.classloader.HotReloadingClassLoader;
 import restx.common.RestxConfig;
 import restx.common.metrics.api.MetricRegistry;
-import restx.factory.*;
+import restx.factory.Factory;
+import restx.factory.Name;
+import restx.factory.NamedComponent;
+import restx.factory.SingletonFactoryMachine;
+import restx.factory.Warehouse;
 import restx.http.HttpStatus;
 import restx.security.RestxSessionCookieFilter;
 import restx.server.WebServer;
@@ -578,7 +587,7 @@ private boolean useHotCompile() {
                 logger.info("can't enable hot compile: restx.app.package is not set.\n" +
                         "Run your app with -Drestx.app.package=<app.base.package> to enable hot compile.");
                 return false;
-            } else if (!hasToolsJar()) {
+            } else if (!hasSystemJavaCompiler()) {
                 logger.info("can't enable hot compile: tools.jar is not in classpath.\n" +
                         "Run your app with a JDK rather than a JRE to enable hot compile.");
                 return false;
@@ -590,7 +599,7 @@ private boolean useHotCompile() {
                 && !getMode().equals(RestxContext.Modes.PROD)
                 && !getMode().equals(RestxContext.Modes.TEST)
                 && appSettings.appPackage().isPresent()
-                && hasToolsJar();
+                && hasSystemJavaCompiler();
         }
     }
 
@@ -602,13 +611,8 @@ private String getMode(RestxRequest restxRequest) {
         return restxRequest.getHeader("RestxMode").or(getMode());
     }
 
-    private boolean hasToolsJar() {
-        try {
-            Class.forName("javax.tools.ToolProvider");
-            return true;
-        } catch (Exception e) {
-            return false;
-        }
+    private boolean hasSystemJavaCompiler() {
+        return Apps.hasSystemJavaCompiler();
     }
 
     private boolean useAutoCompile() {