Add warmup functionality for the servers/clients (#1455)

The warmup triggers an initialisation of the event loop groups, the host name resolver,
loads the necessary native libraries for the transport and native libraries
for the security if security is enabled.

HttpClient:
When the URL scheme is HTTPS and there is no security configured,
the default security will be used thus always try to load the OpenSsl natives
see HttpClientConnect.MonoHttpConnect#subscribe
The URL is parsed on request so the scheme is not known in advance
in order to use this information in the warmup.
With one and the same HttpClient you may trigger various requests

HttpClient client = HttpClient.create();
...
client.get().uri("https://abc.com/")
...
client.get().uri("http://qwe.com/")

Related to #560, #1023, #1425

docs/asciidoc/http-client.adoc

@@ -55,6 +55,31 @@ include::{examplesdir}/address/Application.java[lines=18..33]
 <2> Configures the `HTTP` port
 ====
 
+== Eager Initialization
+
+By default, the initialization of the `HttpClient` resources happens on demand. This means that the `first
+request` absorbs the extra time needed to initialize and load:
+
+ * the event loop group
+ * the host name resolver
+ * the native transport libraries (when native transport is used)
+ * the native libraries for the security (in case of `OpenSsl`)
+
+When you need to preload these resources, you can configure the `HttpClient` as follows:
+
+====
+[source,java,indent=0]
+.{examplesdir}/warmup/Application.java
+----
+include::{examplesdir}/warmup/Application.java[lines=18..36]
+----
+<1> Initialize and load the event loop group, the host name resolver, the native transport libraries and
+the native libraries for the security
+<2> Host name resolution happens with the first request.
+In this example, a connection pool is used, so with the first request the connection to the URL is established,
+the subsequent requests to the same URL reuse the connections from the pool.
+====
+
 == Writing Data
 
 To send data to a given `HTTP` endpoint, you can provide a `Publisher` by using the

docs/asciidoc/http-server.adoc

@@ -47,6 +47,26 @@ include::{examplesdir}/address/Application.java[lines=18..33]
 <2> Configures the `HTTP` server port
 ====
 
+== Eager Initialization
+
+By default, the initialization of the `HttpServer` resources happens on demand. This means that the `bind
+operation` absorbs the extra time needed to initialize and load:
+
+ * the event loop groups
+ * the native transport libraries (when native transport is used)
+ * the native libraries for the security (in case of `OpenSsl`)
+
+When you need to preload these resources, you can configure the `HttpServer` as follows:
+
+====
+[source,java,indent=0]
+.{examplesdir}/warmup/Application.java
+----
+include::{examplesdir}/warmup/Application.java[lines=18..36]
+----
+<1> Initialize and load the event loop groups, the native transport libraries and the native libraries for the security
+====
+
 == Routing HTTP
 
 Defining routes for the `HTTP` server requires configuring the provided

docs/asciidoc/tcp-client.adoc

@@ -48,6 +48,29 @@ include::{examplesdir}/address/Application.java[lines=18..33]
 <2> Configures the `TCP` port
 ====
 
+== Eager Initialization
+
+By default, the initialization of the `TcpClient` resources happens on demand. This means that the `connect
+operation` absorbs the extra time needed to initialize and load:
+
+ * the event loop group
+ * the host name resolver
+ * the native transport libraries (when native transport is used)
+ * the native libraries for the security (in case of `OpenSsl`)
+
+When you need to preload these resources, you can configure the `TcpClient` as follows:
+
+====
+[source,java,indent=0]
+.{examplesdir}/warmup/Application.java
+----
+include::{examplesdir}/warmup/Application.java[lines=18..39]
+----
+<1> Initialize and load the event loop group, the host name resolver, the native transport libraries and
+the native libraries for the security
+<2> Host name resolution happens when connecting to the remote peer
+====
+
 == Writing Data
 
 To send data to a given endpoint, you must attach an I/O handler.

docs/asciidoc/tcp-server.adoc

@@ -47,6 +47,26 @@ include::{examplesdir}/address/Application.java[lines=18..33]
 <2> Configures the `TCP` server port
 ====
 
+== Eager Initialization
+
+By default, the initialization of the `TcpServer` resources happens on demand. This means that the `bind
+operation` absorbs the extra time needed to initialize and load:
+
+ * the event loop groups
+ * the native transport libraries (when native transport is used)
+ * the native libraries for the security (in case of `OpenSsl`)
+
+When you need to preload these resources, you can configure the `TcpServer` as follows:
+
+====
+[source,java,indent=0]
+.{examplesdir}/warmup/Application.java
+----
+include::{examplesdir}/warmup/Application.java[lines=18..36]
+----
+<1> Initialize and load the event loop groups, the native transport libraries and the native libraries for the security
+====
+
 == Writing Data
 
 In order to send data to a connected client, you must attach an I/O handler.

docs/asciidoc/udp-client.adoc

@@ -46,6 +46,27 @@ include::{examplesdir}/address/Application.java[lines=18..34]
 <2> Configures the `port` to which this client should connect
 ====
 
+== Eager Initialization
+
+By default, the initialization of the `UdpClient` resources happens on demand. This means that the `connect
+operation` absorbs the extra time needed to initialize and load:
+
+ * the event loop group
+ * the host name resolver
+ * the native transport libraries (when native transport is used)
+
+When you need to preload these resources, you can configure the `UdpClient` as follows:
+
+====
+[source,java,indent=0]
+.{examplesdir}/warmup/Application.java
+----
+include::{examplesdir}/warmup/Application.java[lines=18..40]
+----
+<1> Initialize and load the event loop group, the host name resolver, and the native transport libraries
+<2> Host name resolution happens when connecting to the remote peer
+====
+
 == Writing Data
 
 To send data to a given peer, you must attach an I/O handler.

docs/asciidoc/udp-server.adoc

@@ -46,6 +46,25 @@ include::{examplesdir}/address/Application.java[lines=18..34]
 <2> Configures the `UDP` server port
 ====
 
+== Eager Initialization
+
+By default, the initialization of the `UdpServer` resources happens on demand. This means that the `bind
+operation` absorbs the extra time needed to initialize and load:
+
+ * the event loop group
+ * the native transport libraries (when native transport is used)
+
+When you need to preload these resources, you can configure the `UdpServer` as follows:
+
+====
+[source,java,indent=0]
+.{examplesdir}/warmup/Application.java
+----
+include::{examplesdir}/warmup/Application.java[lines=18..51]
+----
+<1> Initialize and load the event loop group and the native transport libraries
+====
+
 == Writing Data
 
 To send data to the remote peer, you must attach an I/O handler.

reactor-netty-core/build.gradle

@@ -104,6 +104,10 @@ dependencies {
 	}
 
 	// Testing
+
+	// JSR-305 annotations
+	testCompileOnly "com.google.code.findbugs:jsr305:$jsr305Version"
+
 	testCompile "com.fasterxml.jackson.core:jackson-databind:$jacksonDatabindVersion"
 	testCompile "io.projectreactor:reactor-test:$testAddonVersion"
 	testCompile "org.assertj:assertj-core:$assertJVersion"

reactor-netty-core/src/main/java/reactor/netty/tcp/TcpClient.java

@@ -27,6 +27,8 @@
 import io.netty.channel.ChannelOption;
 import io.netty.channel.EventLoopGroup;
 import io.netty.handler.logging.LogLevel;
+import io.netty.handler.ssl.JdkSslContext;
+import io.netty.handler.ssl.OpenSsl;
 import io.netty.handler.ssl.SslContext;
 import io.netty.resolver.AddressResolverGroup;
 import io.netty.util.AttributeKey;
@@ -355,6 +357,31 @@ public TcpClient secure(SslProvider sslProvider) {
 		return dup;
 	}
 
+	/**
+	 * Based on the actual configuration, returns a {@link Mono} that triggers:
+	 * <ul>
+	 *     <li>an initialization of the event loop group</li>
+	 *     <li>an initialization of the host name resolver</li>
+	 *     <li>loads the necessary native libraries for the transport</li>
+	 *     <li>loads the necessary native libraries for the security if there is such</li>
+	 * </ul>
+	 * By default, when method is not used, the {@code connect operation} absorbs the extra time needed to load resources.
+	 *
+	 * @return a {@link Mono} representing the completion of the warmup
+	 * @since 1.0.3
+	 */
+	@Override
+	public Mono<Void> warmup() {
+		return Mono.when(
+				super.warmup(),
+				Mono.fromRunnable(() -> {
+					SslProvider provider = configuration().sslProvider();
+					if (provider != null && !(provider.getSslContext() instanceof JdkSslContext)) {
+						OpenSsl.version();
+					}
+				}));
+	}
+
 	@Override
 	public TcpClient wiretap(boolean enable) {
 		return super.wiretap(enable);

reactor-netty-core/src/main/java/reactor/netty/tcp/TcpServer.java

@@ -25,6 +25,8 @@
 import io.netty.channel.EventLoopGroup;
 import io.netty.channel.group.ChannelGroup;
 import io.netty.handler.logging.LogLevel;
+import io.netty.handler.ssl.JdkSslContext;
+import io.netty.handler.ssl.OpenSsl;
 import io.netty.handler.ssl.SslContext;
 import io.netty.handler.ssl.util.SelfSignedCertificate;
 import org.reactivestreams.Publisher;
@@ -219,6 +221,30 @@ public TcpServer secure(SslProvider sslProvider) {
 		return dup;
 	}
 
+	/**
+	 * Based on the actual configuration, returns a {@link Mono} that triggers:
+	 * <ul>
+	 *     <li>an initialization of the event loop groups</li>
+	 *     <li>loads the necessary native libraries for the transport</li>
+	 *     <li>loads the necessary native libraries for the security if there is such</li>
+	 * </ul>
+	 * By default, when method is not used, the {@code bind operation} absorbs the extra time needed to load resources.
+	 *
+	 * @return a {@link Mono} representing the completion of the warmup
+	 * @since 1.0.3
+	 */
+	@Override
+	public Mono<Void> warmup() {
+		return Mono.when(
+				super.warmup(),
+				Mono.fromRunnable(() -> {
+					SslProvider provider = configuration().sslProvider();
+					if (provider != null && !(provider.getSslContext() instanceof JdkSslContext)) {
+						OpenSsl.version();
+					}
+				}));
+	}
+
 	@Override
 	public TcpServer wiretap(boolean enable) {
 		return super.wiretap(enable);

reactor-netty-core/src/main/java/reactor/netty/transport/ClientTransport.java

@@ -309,4 +309,26 @@ public T runOn(LoopResources loopResources, boolean preferNative) {
 		}
 		return dup;
 	}
+
+	/**
+	 * Based on the actual configuration, returns a {@link Mono} that triggers:
+	 * <ul>
+	 *     <li>an initialization of the event loop group</li>
+	 *     <li>an initialization of the host name resolver</li>
+	 *     <li>loads the necessary native libraries for the transport</li>
+	 * </ul>
+	 * By default, when method is not used, the {@code connect operation} absorbs the extra time needed to initialize and
+	 * load the resources.
+	 *
+	 * @return a {@link Mono} representing the completion of the warmup
+	 * @since 1.0.3
+	 */
+	public Mono<Void> warmup() {
+		return Mono.fromRunnable(() -> {
+			configuration().eventLoopGroup();
+
+			// By default the host name resolver uses the event loop group configured on client level
+			configuration().resolverInternal();
+		});
+	}
 }

reactor-netty-core/src/main/java/reactor/netty/transport/ServerTransport.java

@@ -322,6 +322,27 @@ public T port(int port) {
 		return bindAddress(() -> AddressUtils.updatePort(configuration().bindAddress(), port));
 	}
 
+	/**
+	 * Based on the actual configuration, returns a {@link Mono} that triggers:
+	 * <ul>
+	 *     <li>an initialization of the event loop groups</li>
+	 *     <li>loads the necessary native libraries for the transport</li>
+	 * </ul>
+	 * By default, when method is not used, the {@code bind operation} absorbs the extra time needed to load resources.
+	 *
+	 * @return a {@link Mono} representing the completion of the warmup
+	 * @since 1.0.3
+	 */
+	public Mono<Void> warmup() {
+		return Mono.fromRunnable(() -> {
+			// event loop group for the server
+			configuration().childEventLoopGroup();
+
+			// event loop group for the server selector
+			configuration().eventLoopGroup();
+		});
+	}
+
 	static final Logger log = Loggers.getLogger(ServerTransport.class);
 
 	static class Acceptor extends ChannelInboundHandlerAdapter {

reactor-netty-core/src/main/java/reactor/netty/udp/UdpServer.java

@@ -265,6 +265,21 @@ public final UdpServer runOn(LoopResources loopResources, InternetProtocolFamily
 		return dup;
 	}
 
+	/**
+	 * Based on the actual configuration, returns a {@link Mono} that triggers:
+	 * <ul>
+	 *     <li>an initialization of the event loop group</li>
+	 *     <li>loads the necessary native libraries for the transport</li>
+	 * </ul>
+	 * By default, when method is not used, the {@code bind operation} absorbs the extra time needed to load resources.
+	 *
+	 * @return a {@link Mono} representing the completion of the warmup
+	 * @since 1.0.3
+	 */
+	public final Mono<Void> warmup() {
+		return Mono.fromRunnable(() -> configuration().eventLoopGroup());
+	}
+
 	@Override
 	public final UdpServer wiretap(boolean enable) {
 		return super.wiretap(enable);