Create a buffer from a byte[]

[source,java]
----
byte[] bytes = new byte[] {1, 3, 5};
Buffer buff = Buffer.buffer(bytes);
----The event bus can be configured. It is particularly useful when the event bus is clustered.
Under the hood the event bus uses TCP connections to send and receive messages, so the `link:../../apidocs/io/vertx/core/eventbus/EventBusOptions.html[EventBusOptions]` let you configure all aspects of these TCP connections.
As the event bus acts as a server and client, the configuration is close to `link:../../apidocs/io/vertx/core/net/NetClientOptions.html[NetClientOptions]` and `link:../../apidocs/io/vertx/core/net/NetServerOptions.html[NetServerOptions]`.

[source,java]
----
VertxOptions options = new VertxOptions()
    .setEventBusOptions(new EventBusOptions()
        .setSsl(true)
        .setKeyStoreOptions(new JksOptions().setPath("keystore.jks").setPassword("wibble"))
        .setTrustStoreOptions(new JksOptions().setPath("keystore.jks").setPassword("wibble"))
        .setClientAuth(ClientAuth.REQUIRED)
    );

Vertx
  .clusteredVertx(options)
  .onComplete(res -> {
    if (res.succeeded()) {
      Vertx vertx = res.result();
      EventBus eventBus = vertx.eventBus();
      System.out.println("We now have a clustered event bus: " + eventBus);
    } else {
      System.out.println("Failed: " + res.cause());
    }
  });
----

The previous snippet depicts how you can use SSL connections for the event bus, instead of plain TCP connections.

**WARNING**: to enforce the security in clustered mode, you **must** configure the cluster manager to use encryption or enforce security.
Refer to the documentation of the cluster manager for further details.

The event bus configuration needs to be consistent in all the cluster nodes.

The `link:../../apidocs/io/vertx/core/eventbus/EventBusOptions.html[EventBusOptions]` also lets you specify whether or not the event bus is clustered, the port and host.

When used in containers, you can also configure the public host and port:

[source,java]
----
VertxOptions options = new VertxOptions()
    .setEventBusOptions(new EventBusOptions()
        .setClusterPublicHost("whatever")
        .setClusterPublicPort(1234)
    );

Vertx
  .clusteredVertx(options)
  .onComplete(res -> {
    if (res.succeeded()) {
      Vertx vertx = res.result();
      EventBus eventBus = vertx.eventBus();
      System.out.println("We now have a clustered event bus: " + eventBus);
    } else {
      System.out.println("Failed: " + res.cause());
    }
  });
----[source,java]
----
Vertx vertx = Vertx.vertx(new VertxOptions().
  setPreferNativeTransport(true)
);

// True when native is available
boolean usingNative = vertx.isNativeTransportEnabled();
System.out.println("Running with native: " + usingNative);
----

NOTE: preferring native transport will not prevent the application to execute (for example if a JAR is missing).
If your application requires native transport, you need to check `link:../../apidocs/io/vertx/core/Vertx.html#isNativeTransportEnabled--[isNativeTransportEnabled]`.If you are using Maven or Gradle, add the following dependency to the _dependencies_ section of your
project descriptor to access the Vert.x Core API:

* Maven (in your `pom.xml`):

[source,xml,subs="+attributes"]
----
<dependency>
 <groupId>io.vertx</groupId>
 <artifactId>vertx-core</artifactId>
 <version>4.5.5</version>
</dependency>
----

* Gradle (in your `build.gradle` file):

[source,groovy,subs="+attributes"]
----
dependencies {
 compile 'io.vertx:vertx-core:4.5.5'
}
----=== Error handling

As you saw in previous sections the DnsClient allows you to pass in a Handler which will be notified with an
AsyncResult once the query was complete. In case of an error it will be notified with a DnsException which will
hole a `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html[DnsResponseCode]` that indicate why the resolution failed. This DnsResponseCode
can be used to inspect the cause in more detail.

Possible DnsResponseCodes are:

- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#NOERROR[NOERROR]` No record was found for a given query
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#FORMERROR[FORMERROR]` Format error
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#SERVFAIL[SERVFAIL]` Server failure
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#NXDOMAIN[NXDOMAIN]` Name error
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#NOTIMPL[NOTIMPL]` Not implemented by DNS Server
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#REFUSED[REFUSED]` DNS Server refused the query
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#YXDOMAIN[YXDOMAIN]` Domain name should not exist
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#YXRRSET[YXRRSET]` Resource record should not exist
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#NXRRSET[NXRRSET]` RRSET does not exist
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#NOTZONE[NOTZONE]` Name not in zone
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#BADVERS[BADVERS]` Bad extension mechanism for version
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#BADSIG[BADSIG]` Bad signature
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#BADKEY[BADKEY]` Bad key
- `link:../../apidocs/io/vertx/core/dns/DnsResponseCode.html#BADTIME[BADTIME]` Bad timestamp

All of those errors are "generated" by the DNS Server itself.

You can obtain the DnsResponseCode from the DnsException like:

[source,java]
----
DnsClient client = vertx.createDnsClient(53, "10.0.0.1");
client
  .lookup("nonexisting.vert.xio")
  .onComplete(ar -> {
    if (ar.succeeded()) {
      String record = ar.result();
      System.out.println(record);
    } else {
      Throwable cause = ar.cause();
      if (cause instanceof DnsException) {
        DnsException exception = (DnsException) cause;
        DnsResponseCode code = exception.code();
        // ...
      } else {
        System.out.println("Failed to resolve entry" + ar.cause());
      }
    }
  });
----==== Message Codecs

You can send any object you like across the event bus if you define and register a `link:../../apidocs/io/vertx/core/eventbus/MessageCodec.html[message codec]` for it.

Message codecs have a name and you specify that name in the `link:../../apidocs/io/vertx/core/eventbus/DeliveryOptions.html[DeliveryOptions]`
when sending or publishing the message:

[source,java]
----
eventBus.registerCodec(myCodec);

DeliveryOptions options = new DeliveryOptions().setCodecName(myCodec.name());

eventBus.send("orders", new MyPOJO(), options);
----

If you always want the same codec to be used for a particular type then you can register a default codec for it, then
you don't have to specify the codec on each send in the delivery options:

[source,java]
----
eventBus.registerDefaultCodec(MyPOJO.class, myCodec);

eventBus.send("orders", new MyPOJO());
----

You unregister a message codec with `link:../../apidocs/io/vertx/core/eventbus/EventBus.html#unregisterCodec-java.lang.String-[unregisterCodec]`.

Message codecs don't always have to encode and decode as the same type. For example you can write a codec that
allows a MyPOJO class to be sent, but when that message is sent to a handler it arrives as a MyOtherPOJO class.

Vert.x has built-in codecs for certain data types:

- basic types (string, byte array, byte, int, long, double, boolean, short, char), or
- some Vert.x data types (buffers, JSON array, JSON objects), or
- types implementing the `link:../../apidocs/io/vertx/core/shareddata/ClusterSerializable.html[ClusterSerializable]` interface, or
- types implementing the `java.io.Serializable` interface.

[IMPORTANT]
====
In clustered mode, `link:../../apidocs/io/vertx/core/shareddata/ClusterSerializable.html[ClusterSerializable]` and `java.io.Serializable` objects are rejected by default, for security reasons.

You can define which classes are allowed for encoding and decoding by providing functions which inspect the name of the class:

- `link:../../apidocs/io/vertx/core/eventbus/EventBus.html#clusterSerializableChecker-java.util.function.Function-[EventBus.clusterSerializableChecker()]`, and
- `link:../../apidocs/io/vertx/core/eventbus/EventBus.html#serializableChecker-java.util.function.Function-[EventBus.serializableChecker()]`.
======== Setting headers on messages

Messages sent over the event bus can also contain headers. This can be specified by providing a
`link:../../apidocs/io/vertx/core/eventbus/DeliveryOptions.html[DeliveryOptions]` when sending or publishing:

[source,java]
----
DeliveryOptions options = new DeliveryOptions();
options.addHeader("some-header", "some-value");
eventBus.send("news.uk.sport", "Yay! Someone kicked a ball", options);
----== Host name resolution

Vert.x uses an an address resolver for resolving host name into IP addresses instead of
the JVM built-in blocking resolver.

An host name resolves to an IP address using:

- the _hosts_ file of the operating system
- otherwise DNS queries against a list of servers

By default it will use the list of the system DNS server addresses from the environment, if that list cannot be
retrieved it will use Google's public DNS servers `"8.8.8.8"` and `"8.8.4.4"`.

DNS servers can be also configured when creating a `link:../../apidocs/io/vertx/core/Vertx.html[Vertx]` instance:

[source,java]
----
Vertx vertx = Vertx.vertx(new VertxOptions().
    setAddressResolverOptions(
        new AddressResolverOptions().
            addServer("192.168.0.1").
            addServer("192.168.0.2:40000"))
);
----

The default port of a DNS server is `53`, when a server uses a different port, this port can be set
using a colon delimiter: `192.168.0.2:40000`.

NOTE: sometimes it can be desirable to use the JVM built-in resolver, the JVM system property
_-Dvertx.disableDnsResolver=true_ activates this behavior

=== Failover

When a server does not reply in a timely manner, the resolver will try the next one from the list, the search
is limited by `link:../../apidocs/io/vertx/core/dns/AddressResolverOptions.html#setMaxQueries-int-[setMaxQueries]` (the default value is `4` queries).

A DNS query is considered as failed when the resolver has not received a correct answer within
`link:../../apidocs/io/vertx/core/dns/AddressResolverOptions.html#getQueryTimeout--[getQueryTimeout]` milliseconds (the default value is `5` seconds).

=== Server list rotation

By default the dns server selection uses the first one, the remaining servers are used for failover.

You can configure `link:../../apidocs/io/vertx/core/dns/AddressResolverOptions.html#setRotateServers-boolean-[setRotateServers]` to `true` to let
the resolver perform a round-robin selection instead. It spreads the query load among the servers and avoids
all lookup to hit the first server of the list.

Failover still applies and will use the next server in the list.

=== Hosts mapping

The _hosts_ file of the operating system is used to perform an hostname lookup for an ipaddress.

An alternative _hosts_ file can be used instead:

[source,java]
----
Vertx vertx = Vertx.vertx(new VertxOptions().
    setAddressResolverOptions(
        new AddressResolverOptions().
            setHostsPath("/path/to/hosts"))
);
----

=== Search domains

By default the resolver will use the system DNS search domains from the environment. Alternatively an explicit search domain
list can be provided:

[source,java]
----
Vertx vertx = Vertx.vertx(new VertxOptions().
    setAddressResolverOptions(
        new AddressResolverOptions().addSearchDomain("foo.com").addSearchDomain("bar.com"))
);
----

When a search domain list is used, the threshold for the number of dots is `1` or loaded from `/etc/resolv.conf`
on Linux, it can be configured to a specific value with `link:../../apidocs/io/vertx/core/dns/AddressResolverOptions.html#setNdots-int-[setNdots]`.

=== MacOS configuration

MacOS has a specific native extension to get the name server configuration of the system based on
<a href="https://opensource.apple.com/tarballs/mDNSResponder/">Apple's open source mDNSResponder</a>. When this extension is not present,
Netty logs the following warning.

----
[main] WARN io.netty.resolver.dns.DnsServerAddressStreamProviders - Can not find io.netty.resolver.dns.macos.MacOSDnsServerAddressStreamProvider in the classpath, fallback to system defaults. This may result in incorrect DNS resolutions on MacOS.
----

This extension is not required as its absence does not prevent Vert.x to execute, yet is *recommended*.

You can use add it to your classpath to improve the integration and remove the warning.

.Intel-based Mac
[source,xml]
----
<profile>
 <id>mac-intel</id>
 <activation>
   <os>
     <family>mac</family>
     <arch>x86_64</arch>
   </os>
 </activation>
 <dependencies>
   <dependency>
     <groupId>io.netty</groupId>
     <artifactId>netty-resolver-dns-native-macos</artifactId>
     <classifier>osx-x86_64</classifier>
     <!--<version>Should align with netty version that Vert.x uses</version>-->
   </dependency>
 </dependencies>
</profile>
----

.M1/M2 Mac
[source,xml]
----
<profile>
 <id>mac-silicon</id>
 <activation>
   <os>
     <family>mac</family>
     <arch>aarch64</arch>
   </os>
 </activation>
 <dependencies>
   <dependency>
     <groupId>io.netty</groupId>
     <artifactId>netty-resolver-dns-native-macos</artifactId>
     <classifier>osx-aarch_64</classifier>
     <!--<version>Should align with netty version that Vert.x uses</version>-->
   </dependency>
 </dependencies>
</profile>
----== In the beginning there was Vert.x

You can't do much in Vert.x-land unless you can communicate with a `link:../../apidocs/io/vertx/core/Vertx.html[Vertx]` object!

It's the control centre of Vert.x and is how you do pretty much everything, including creating clients and servers,
getting a reference to the event bus, setting timers, as well as many other things.

So how do you get an instance?

If you're embedding Vert.x then you simply create an instance as follows:

[source,java]
----
Vertx vertx = Vertx.vertx();
----

NOTE: Most applications will only need a single Vert.x instance, but it's possible to create multiple Vert.x instances if you
require, for example, isolation between the event bus or different groups of servers and clients.

=== Specifying options when creating a Vertx object

When creating a Vert.x object you can also specify options if the defaults aren't right for you:

[source,java]
----
Vertx vertx = Vertx.vertx(new VertxOptions().setWorkerPoolSize(40));
----

The `link:../../apidocs/io/vertx/core/VertxOptions.html[VertxOptions]` object has many settings and allows you to configure things like clustering, high availability, pool sizes and various other settings.

=== Creating a clustered Vert.x object

If you're creating a *clustered Vert.x* (See the section on the <<event_bus, event bus>> for more information on clustering the event bus),
then you will normally use the asynchronous variant to create the Vertx object.

This is because it usually takes some time (maybe a few seconds) for the different Vert.x instances in a cluster to group together.
During that time, we don't want to block the calling thread, so we give the result to you asynchronously.== JSON
:toc: left

Unlike some other languages, Java does not have first class support for http://json.org/[JSON] so we provide
two classes to make handling JSON in your Vert.x applications a bit easier.

=== JSON objects

The `link:../../apidocs/io/vertx/core/json/JsonObject.html[JsonObject]` class represents JSON objects.

A JSON object is basically just a map which has string keys and values can be of one of the JSON supported types
(string, number, boolean).

JSON objects also support null values.

==== Creating JSON objects

Empty JSON objects can be created with the default constructor.

You can create a JSON object from a string JSON representation as follows:

[source,java]
----
String jsonString = "{\"foo\":\"bar\"}";
JsonObject object = new JsonObject(jsonString);
----

You can create a JSON object from a map as follows:

[source,java]
----
Map<String, Object> map = new HashMap<>();
map.put("foo", "bar");
map.put("xyz", 3);
JsonObject object = new JsonObject(map);
----

==== Putting entries into a JSON object

Use the `link:../../apidocs/io/vertx/core/json/JsonObject.html#put-java.lang.String-java.lang.Object-[put]` methods to put values into the JSON object.

The method invocations can be chained because of the fluent API:

[source,java]
----
JsonObject object = new JsonObject();
object.put("foo", "bar").put("num", 123).put("mybool", true);
----

==== Getting values from a JSON object

You get values from a JSON object using the `getXXX` methods, for example:

[source,java]
----
String val = jsonObject.getString("some-key");
int intVal = jsonObject.getInteger("some-other-key");
----

==== Mapping between JSON objects and Java objects

You can create a JSON object from the fields of a Java object as follows:

You can instantiate a Java object and populate its fields from a JSON object as follows:

[source,java]
----
request.bodyHandler(buff -> {
  JsonObject jsonObject = buff.toJsonObject();
  User javaObject = jsonObject.mapTo(User.class);
});
----

Note that both of the above mapping directions use Jackson's `ObjectMapper#convertValue()` to perform the
mapping. See the Jackson documentation for information on the impact of field and constructor visibility, caveats
on serialization and deserialization across object references, etc.

However, in the simplest case, both `mapFrom` and `mapTo` should succeed if all fields of the Java class are
public (or have public getters/setters), and if there is a public default constructor (or no defined constructors).

Referenced objects will be transitively serialized/deserialized to/from nested JSON objects as
long as the object graph is acyclic.

==== Encoding a JSON object to a String

You use `link:../../apidocs/io/vertx/core/json/JsonObject.html#encode--[encode]` to encode the object to a String form.

=== JSON arrays

The `link:../../apidocs/io/vertx/core/json/JsonArray.html[JsonArray]` class represents JSON arrays.

A JSON array is a sequence of values (string, number, boolean).

JSON arrays can also contain null values.

==== Creating JSON arrays

Empty JSON arrays can be created with the default constructor.

You can create a JSON array from a string JSON representation as follows:

[source,java]
----
String jsonString = "[\"foo\",\"bar\"]";
JsonArray array = new JsonArray(jsonString);
----

==== Adding entries into a JSON array

You add entries to a JSON array using the `link:../../apidocs/io/vertx/core/json/JsonArray.html#add-java.lang.Object-[add]` methods.

[source,java]
----
JsonArray array = new JsonArray();
array.add("foo").add(123).add(false);
----

==== Getting values from a JSON array

You get values from a JSON array using the `getXXX` methods, for example:

[source,java]
----
String val = array.getString(0);
Integer intVal = array.getInteger(1);
Boolean boolVal = array.getBoolean(2);
----

==== Encoding a JSON array to a String

You use `link:../../apidocs/io/vertx/core/json/JsonArray.html#encode--[encode]` to encode the array to a String form.

==== Creating arbitrary JSON

Creating JSON object and array assumes you are using valid string representation.

When you are unsure of the string validity then you should use instead ``link:../../apidocs/io/vertx/core/json/Json.html#decodeValue-java.lang.String-[Json.decodeValue]``

[source,java]
----
Object object = Json.decodeValue(arbitraryJson);
if (object instanceof JsonObject) {
  // That's a valid json object
} else if (object instanceof JsonArray) {
  // That's a valid json array
} else if (object instanceof String) {
  // That's valid string
} else {
  // etc...
}
----=== Passing configuration to a verticle

Configuration in the form of JSON can be passed to a verticle at deployment time:

[source,java]
----
JsonObject config = new JsonObject().put("name", "tim").put("directory", "/blah");
DeploymentOptions options = new DeploymentOptions().setConfig(config);
vertx.deployVerticle("com.mycompany.MyOrderProcessorVerticle", options);
----

This configuration is then available via the `link:../../apidocs/io/vertx/core/Context.html[Context]` object or directly using the
`link:../../apidocs/io/vertx/core/AbstractVerticle.html#config--[config]` method. The configuration is returned as a JSON object so you
can retrieve data as follows:

[source,java]
----
System.out.println("Configuration: " + config().getString("name"));
----

=== Accessing environment variables in a Verticle

Environment variables and system properties are accessible using the Java API:

[source,java]
----
System.getProperty("prop");
System.getenv("HOME");
----=== Writing Verticles

Verticle classes must implement the `link:../../apidocs/io/vertx/core/Verticle.html[Verticle]` interface.

They can implement it directly if you like but usually it's simpler to extend
the abstract class `link:../../apidocs/io/vertx/core/AbstractVerticle.html[AbstractVerticle]`.

Here's an example verticle:

----
public class MyVerticle extends AbstractVerticle {

 // Called when verticle is deployed
 public void start() {
 }

 // Optional - called when verticle is undeployed
 public void stop() {
 }

}
----

Normally you would override the start method like in the example above.

When Vert.x deploys the verticle it will call the start method, and when the method has completed the verticle will
be considered started.

You can also optionally override the stop method. This will be called by Vert.x when the verticle is undeployed and when
the method has completed the verticle will be considered stopped.

=== Asynchronous Verticle start and stop

Sometimes you want to do something in your verticle start-up which takes some time and you don't want the verticle to
be considered deployed until that happens. For example you might want to start an HTTP server in the start method and
propagate the asynchronous result of the server `listen` method.

You can't block waiting for the HTTP server to bind in your start method as that would break the <<golden_rule, Golden Rule>>.

So how can you do this?

The way to do it is to implement the *asynchronous* start method. This version of the method takes a Future as a parameter.
When the method returns the verticle will *not* be considered deployed.

Some time later, after you've done everything you need to do (e.g. start the HTTP server), you can call complete
on the Future (or fail) to signal that you're done.

Here's an example:

----
public class MyVerticle extends AbstractVerticle {

 private HttpServer server;

 public void start(Promise<Void> startPromise) {
   server = vertx.createHttpServer().requestHandler(req -> {
     req.response()
       .putHeader("content-type", "text/plain")
       .end("Hello from Vert.x!");
     });

   // Now bind the server:
   server.listen(8080, res -> {
     if (res.succeeded()) {
       startPromise.complete();
     } else {
       startPromise.fail(res.cause());
     }
   });
 }
}
----

Similarly, there is an asynchronous version of the stop method too. You use this if you want to do some verticle
cleanup that takes some time.

----
public class MyVerticle extends AbstractVerticle {

 public void start() {
   // Do something
 }

 public void stop(Promise<Void> stopPromise) {
   obj.doSomethingThatTakesTime(res -> {
     if (res.succeeded()) {
       stopPromise.complete();
     } else {
       stopPromise.fail();
     }
   });
 }
}
----

INFO: You don't need to manually stop the HTTP server started by a verticle, in the verticle's stop method. Vert.x
will automatically stop any running server when the verticle is undeployed.