Add ServerImpl#awaitTerminated() method. Fixes #80

core/src/main/java/io/grpc/ServerImpl.java

@@ -200,6 +200,15 @@ public class ServerImpl implements Server {
     return terminated;
   }
 
+  /**
+   * Waits for the server to become terminated.
+   */
+  public synchronized void awaitTerminated() throws InterruptedException {
+    while(!terminated) {
+      wait();
+    }
+  }
+
   /**
    * Returns whether the server is terminated. Terminated servers have no running calls and
    * relevant resources released (like TCP connections).

netty/src/main/java/io/grpc/transport/netty/NettyServerBuilder.java

@@ -40,6 +40,7 @@ import io.grpc.SharedResourceHolder;
 import io.grpc.transport.ServerListener;
 import io.netty.channel.EventLoopGroup;
 import io.netty.handler.ssl.SslContext;
+import io.grpc.ServerImpl;
 
 import java.net.InetSocketAddress;
 import java.net.SocketAddress;
@@ -88,6 +89,15 @@ public final class NettyServerBuilder extends AbstractServerBuilder<NettyServerB
    *
    * <p>The server won't take ownership of the given EventLoopGroup. It's caller's responsibility
    * to shut it down when it's desired.
+   *
+   * <p>Grpc uses non-daemon {@link Thread}s by default and thus a {@link ServerImpl} will
+   * continue to run even after the main thread has terminated. However, users have to be cautious
+   * when providing their own {@link EventLoopGroup}s.
+   * For example, Netty's {@link EventLoopGroup}s use daemon threads by default
+   * and thus an application with only daemon threads running besides the main thread will exit as
+   * soon as the main thread completes.
+   * A simple solution to this problem is to call {@link ServerImpl#awaitTerminated()} to
+   * keep the main thread alive until the server has terminated.
    */
   public NettyServerBuilder userBossEventLoopGroup(EventLoopGroup group) {
     this.userBossEventLoopGroup = group;
@@ -102,6 +112,15 @@ public final class NettyServerBuilder extends AbstractServerBuilder<NettyServerB
    *
    * <p>The server won't take ownership of the given EventLoopGroup. It's caller's responsibility
    * to shut it down when it's desired.
+   *
+   * <p>Grpc uses non-daemon {@link Thread}s by default and thus a {@link ServerImpl} will
+   * continue to run even after the main thread has terminated. However, users have to be cautious
+   * when providing their own {@link EventLoopGroup}s.
+   * For example, Netty's {@link EventLoopGroup}s use daemon threads by default
+   * and thus an application with only daemon threads running besides the main thread will exit as
+   * soon as the main thread completes.
+   * A simple solution to this problem is to call {@link ServerImpl#awaitTerminated()} to
+   * keep the main thread alive until the server has terminated.
    */
   public NettyServerBuilder workerEventLoopGroup(EventLoopGroup group) {
     this.userWorkerEventLoopGroup = group;