////
This guide is maintained in the main Quarkus repository
and pull requests should be submitted there:
https://github.com/quarkusio/quarkus/tree/main/docs/src/main/asciidoc
////
= Contexts and Dependency Injection

include::./attributes.adoc[]
:numbered:
:sectnums:
:sectnumlevels: 4
:toc:

Quarkus DI solution (also called ArC) is based on the https://jakarta.ee/specifications/cdi/2.0/cdi-spec-2.0.html[Contexts and Dependency Injection for Java 2.0, window="_blank"] specification.
However, it is not a full CDI implementation verified by the TCK.
Only a subset of the CDI features is implemented - see also <<supported_features,the list of supported features>> and <<limitations,the list of limitations>>.

TIP: If you're new to CDI then we recommend you to read the xref:cdi.adoc[Introduction to CDI] first. 

NOTE: Most of the existing CDI code should work just fine but there are some small differences which follow from the Quarkus architecture and goals.

[[bean_discovery]]
== Bean Discovery

Bean discovery in CDI is a complex process which involves legacy deployment structures and accessibility requirements of the underlying module architecture.
However, Quarkus is using a *simplified bean discovery*.
There is only single bean archive with the https://jakarta.ee/specifications/cdi/2.0/cdi-spec-2.0.html#default_bean_discovery[bean discovery mode `annotated`, window="_blank"] and no visibility boundaries.

The bean archive is synthesized from:

* the application classes,
* dependencies that contain a `beans.xml` descriptor (content is ignored),
* dependencies that contain a Jandex index - `META-INF/jandex.idx`,
* dependencies referenced by `quarkus.index-dependency` in `application.properties`,
* and Quarkus integration code.

Bean classes that don't have a https://jakarta.ee/specifications/cdi/2.0/cdi-spec-2.0.html#bean_defining_annotations[bean defining annotation, window="_blank"] are not discovered.
This behavior is defined by CDI.
But producer methods and fields and observer methods are discovered even if the declaring class is not annotated with a bean defining annotation (this behavior is different to what is defined in CDI).
In fact, the declaring bean classes are considered annotated with `@Dependent`. 

NOTE: Quarkus extensions may declare additional discovery rules. For example, `@Scheduled` business methods are registered even if the declaring class is not annotated with a bean defining annotation.

=== How to Generate a Jandex Index

A dependency with a Jandex index is automatically scanned for beans.
To generate the index just add the following to your `pom.xml`:

[source,xml,subs="attributes+"]
----
<build>
  <plugins>
    <plugin>
      <groupId>org.jboss.jandex</groupId>
      <artifactId>jandex-maven-plugin</artifactId>
      <version>{jandex-maven-plugin-version}</version>
      <executions>
        <execution>
          <id>make-index</id>
          <goals>
            <goal>jandex</goal>
          </goals>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>
----

If you are are using gradle, you can apply the following plugin to your `build.gradle`:

[source,groovy]
----
plugins {
    id 'org.kordamp.gradle.jandex' version '0.11.0'
}
----

If you can't modify the dependency, you can still index it by adding `quarkus.index-dependency` entries to your `application.properties`:

[source,properties]
----
quarkus.index-dependency.<name>.group-id=
quarkus.index-dependency.<name>.artifact-id=
quarkus.index-dependency.<name>.classifier=(this one is optional)
----

For example, the following entries ensure that the `org.acme:acme-api` dependency is indexed:

.Example application.properties
[source,properties]
----
quarkus.index-dependency.acme.group-id=org.acme <1>
quarkus.index-dependency.acme.artifact-id=acme-api <2>
----
<1> Value is a group id for a dependency identified by name `acme`.
<2> Value is an artifact id for a dependency identified by name `acme`.

=== How To Exclude Types and Dependencies from Discovery

It may happen that some beans from third-party libraries do not work correctly in Quarkus.
A typical example is a bean injecting a portable extension.
In such case, it's possible to exclude types and dependencies from the bean discovery.
The `quarkus.arc.exclude-types` property accepts a list of string values that are used to match classes that should be excluded.

.Value Examples
|===
|Value|Description
|`org.acme.Foo`| Match the fully qualified name of the class
|`org.acme.*`| Match classes with package `org.acme`
|`org.acme.**`| Match classes where the package starts with `org.acme` 
|`Bar`| Match the simple name of the class
|===

.Example application.properties
[source,properties]
----
quarkus.arc.exclude-types=org.acme.Foo,org.acme.*,Bar <1><2><3>
----
<1> Exclude the type `org.acme.Foo`.
<2> Exclude all types from the `org.acme` package.
<3> Exclude all types whose simple name is `Bar`

It is also possible to exclude a dependency artifact that would be otherwise scanned for beans.
For example, because it contains a `beans.xml` descriptor.  

.Example application.properties
[source,properties]
----
quarkus.arc.exclude-dependency.acme.group-id=org.acme <1>
quarkus.arc.exclude-dependency.acme.artifact-id=acme-services <2>
----
<1> Value is a group id for a dependency identified by name `acme`.
<2> Value is an artifact id for a dependency identified by name `acme`.

== Native Executables and Private Members

Quarkus is using GraalVM to build a native executable.
One of the limitations of GraalVM is the usage of https://github.com/oracle/graal/blob/master/docs/reference-manual/native-image/Limitations.md#reflection[Reflection, window="_blank"].
Reflective operations are supported but all relevant members must be registered for reflection explicitly.
Those registrations result in a bigger native executable.

And if Quarkus DI needs to access a private member it *has to use reflection*.
That's why Quarkus users are encouraged __not to use private members__ in their beans.
This involves injection fields, constructors and initializers, observer methods, producer methods and fields, disposers and interceptor methods.

How to avoid using private members?
You can use package-private modifiers:

[source,java]
----
@ApplicationScoped
public class CounterBean {

    @Inject
    CounterService counterService; <1>
    
    void onMessage(@Observes Event msg) { <2>
    }
}
----
<1> A package-private injection field.
<2> A package-private observer method.

Or constructor injection:

[source,java]
----
@ApplicationScoped
public class CounterBean {

    private CounterService service;

    CounterBean(CounterService service) { <1>
      this.service = service;
    }
}
----
<1> A package-private constructor injection. `@Inject` is optional in this particular case.

[[supported_features]]
== Supported Features

* Programming model
** Managed beans implemented by a Java class
*** `@PostConstruct` and `@PreDestroy` lifecycle callbacks
** Producer methods and fields, disposers
** Qualifiers
** Alternatives
** Stereotypes
* Dependency injection and lookup
** Field, constructor and initializer/setter injection
** Type-safe resolution 
** Programmatic lookup via `javax.enterprise.inject.Instance`
** Client proxies
** Injection point metadata
* Scopes and contexts
** `@Dependent`, `@ApplicationScoped`, `@Singleton`, `@RequestScoped` and `@SessionScoped`
** Custom scopes and contexts
* Interceptors
** Business method interceptors: `@AroundInvoke`
** Interceptors for lifecycle event callbacks: `@PostConstruct`, `@PreDestroy`, `@AroundConstruct`
* Decorators
* Events and observer methods, including asynchronous events and transactional observer methods

[[limitations]]
== Limitations

* `@ConversationScoped` is not supported
* Portable Extensions are not supported
* `BeanManager` - only the following methods are implemented: `getBeans()`, `createCreationalContext()`, `getReference()`, `getInjectableReference()` , `resolve()`, `getContext()`, `fireEvent()`, `getEvent()` and `createInstance()`
* Specialization is not supported
* `beans.xml` descriptor content is ignored
* Passivation and passivating scopes are not supported
* Interceptor methods on superclasses are not implemented yet
* `@Interceptors` is not supported

[[nonstandard_features]]
== Non-standard Features

=== Eager Instantiation of Beans

[[lazy_by_default]]
==== Lazy By Default

By default, CDI beans are created lazily, when needed.
What exactly "needed" means depends on the scope of a bean.

* A *normal scoped bean* (`@ApplicationScoped`, `@RequestScoped`, etc.) is needed when a method is invoked upon an injected instance (contextual reference per the specification). 
+
In other words, injecting a normal scoped bean will not suffice because a _client proxy_ is injected instead of a contextual instance of the bean.

* A *bean with a pseudo-scope* (`@Dependent` and `@Singleton` ) is created when injected.

.Lazy Instantiation Example
[source,java]
----
@Singleton // => pseudo-scope
class AmazingService {
  String ping() {
    return "amazing";
  }
}

@ApplicationScoped // => normal scope
class CoolService {
  String ping() {
    return "cool";
  }
}

@Path("/ping")
public class PingResource {

  @Inject
  AmazingService s1; <1>
  
  @Inject
  CoolService s2; <2>
  
  @GET
  public String ping() {
    return s1.ping() + s2.ping(); <3>
  }
}
----
<1> Injection triggers the instantiation of `AmazingService`.
<2> Injection itself does not result in the instantiation of `CoolService`. A client proxy is injected. 
<3> The first invocation upon the injected proxy triggers the instantiation of `CoolService`.

[[startup_event]]
==== Startup Event

However, if you really need to instantiate a bean eagerly you can:

* Declare an observer of the `StartupEvent` - the scope of the bean does not matter in this case:
+
[source,java]
----
@ApplicationScoped
class CoolService {
  void startup(@Observes StartupEvent event) { <1>
  }
}
----
<1> A `CoolService` is created during startup to service the observer method invocation.

* Use the bean in an observer of the `StartupEvent` - normal scoped beans must be used as described in <<lazy_by_default>>:    
+
[source,java]
----
@Dependent
class MyBeanStarter {

  void startup(@Observes StartupEvent event, AmazingService amazing, CoolService cool) { <1>
    cool.toString(); <2>
  }
}
----
<1> The `AmazingService` is created during injection.
<2> The `CoolService` is a normal scoped bean so we have to invoke a method upon the injected proxy to force the instantiation.

* Annotate the bean with `@io.quarkus.runtime.Startup` as described in xref:lifecycle.adoc#startup_annotation[Startup annotation]:
+
[source,java]
----
@Startup // <1>
@ApplicationScoped
public class EagerAppBean {

   private final String name;

   EagerAppBean(NameGenerator generator) { // <2>
     this.name = generator.createName();
   }
}
----
<1> For each bean annotated with `@Startup` a synthetic observer of `StartupEvent` is generated. The default priority is used.
<2> The bean constructor is called when the application starts and the resulting contextual instance is stored in the application context.

NOTE: Quarkus users are encouraged to always prefer the `@Observes StartupEvent` to `@Initialized(ApplicationScoped.class)` as explained in the xref:lifecycle.adoc[Application Initialization and Termination] guide.

=== Request Context Lifecycle

The request context is also active:

* during notification of a synchronous observer method.

The request context is destroyed:

* after the observer notification completes for an event, if it was not already active when the notification started.

NOTE: An event with qualifier `@Initialized(RequestScoped.class)` is fired when the request context is initialized for an observer notification. Moreover, the events with qualifiers `@BeforeDestroyed(RequestScoped.class)` and `@Destroyed(RequestScoped.class)` are fired when the request context is destroyed.

=== Qualified Injected Fields

In CDI, if you declare a field injection point you need to use `@Inject` and optionally a set of qualifiers.

[source,java]
----
  @Inject
  @ConfigProperty(name = "cool")
  String coolProperty;
----

In Quarkus, you can skip the `@Inject` annotation completely if the injected field declares at least one qualifier.

[source,java]
----
  @ConfigProperty(name = "cool")
  String coolProperty;
----

NOTE: With the notable exception of one special case discussed below, `@Inject` is still required for constructor and method injection.

=== Simplified Constructor Injection

In CDI, a normal scoped bean must always declare a no-args constructor (this constructor is normally generated by the compiler unless you declare any other constructor).
However, this requirement complicates constructor injection - you need to provide a dummy no-args constructor to make things work in CDI.

[source,java]
----
@ApplicationScoped
public class MyCoolService {

  private SimpleProcessor processor;

  MyCoolService() { // dummy constructor needed
  }

  @Inject // constructor injection
  MyCoolService(SimpleProcessor processor) {
    this.processor = processor;
  }
}
----

There is no need to declare dummy constructors for normal scoped bean in Quarkus - they are generated automatically.
Also if there's only one constructor there is no need for `@Inject`.

[source,java]
----
@ApplicationScoped
public class MyCoolService {

  private SimpleProcessor processor;

  MyCoolService(SimpleProcessor processor) {
    this.processor = processor;
  }
}
----

NOTE: We don't generate a no-args constructor automatically if a bean class extends a class that does not declare a no-args constructor.

[[remove_unused_beans]]
=== Removing Unused Beans

The container attempts to remove all unused beans, interceptors and decorators during build by default.
This optimization helps to minimize the amount of generated classes, thus conserving memory.
However, Quarkus can't detect the programmatic lookup performed via the `CDI.current()` static method.
Therefore, it is possible that a removal results in a false positive error, i.e. a bean is removed although it's actually used.
In such cases, you'll notice a big warning in the log.
Users and extension authors have several options <<eliminate_false_positives,how to eliminate false positives>>.

The optimization can be disabled by setting `quarkus.arc.remove-unused-beans` to `none` or `false`.
Quarkus also provides a middle ground where application beans are never removed whether or not they are unused, while the optimization proceeds normally for non application classes. 
To use this mode, set `quarkus.arc.remove-unused-beans` to `fwk` or `framework`.

==== What's Removed?

Quarkus first identifies so-called _unremovable_ beans that form the roots in the dependency tree.
A good example is a JAX-RS resource class or a bean which declares a `@Scheduled` method.

An _unremovable_ bean:

* is excluded from removal by an extension, or
* has a name designated via `@Named`, or
* declares an observer method.

An _unused_ bean:

* is not _unremovable_, and
* is not eligible for injection to any injection point in the dependency tree, and
* does not declare any producer which is eligible for injection to any injection point in the dependency tree, and
* is not eligible for injection into any `javax.enterprise.inject.Instance` or `javax.inject.Provider` injection point.

Unused interceptors and decorators are not associated with any bean.

[TIP]
====
When using the dev mode (running `./mvnw clean compile quarkus:dev`), you can see more information about which beans are being removed:

1. In the console - just enable the DEBUG level in your `application.properties`, i.e. `quarkus.log.category."io.quarkus.arc.processor".level=DEBUG`
2. In the relevant Dev UI page
====

[[eliminate_false_positives]]
==== How To Eliminate False Positives

Users can instruct the container to not remove any of their specific beans (even if they satisfy all the rules specified above) by annotating them with `@io.quarkus.arc.Unremovable`.
This annotation can be declared on a class, a producer method or field.

Since this is not always possible, there is an option to achieve the same via `application.properties`.
The `quarkus.arc.unremovable-types` property accepts a list of string values that are used to match beans based on their name or package.

.Value Examples
|===
|Value|Description
|`org.acme.Foo`| Match the fully qualified name of the bean class
|`org.acme.*`| Match beans where the package of the bean class is `org.acme`
|`org.acme.**`| Match beans where the package of the bean class starts with `org.acme`
|`Bar`| Match the simple name of the bean class
|===

.Example application.properties
[source,properties]
----
quarkus.arc.unremovable-types=org.acme.Foo,org.acme.*,Bar
----

Furthermore, extensions can eliminate false positives by producing an `UnremovableBeanBuildItem`.

[[default_beans]]
=== Default Beans

Quarkus adds a capability that CDI currently does not support which is to conditionally declare a bean if no other bean with equal types and qualifiers was declared by any available means (bean class, producer, synthetic bean, ...)
This is done using the `@io.quarkus.arc.DefaultBean` annotation and is best explained with an example.

Say there is a Quarkus extension that among other things declares a few CDI beans like the following code does:

[source,java]
----
@Dependent
public class TracerConfiguration {

    @Produces
    public Tracer tracer(Reporter reporter, Configuration configuration) {
        return new Tracer(reporter, configuration);
    }

    @Produces
    @DefaultBean
    public Configuration configuration() {
        // create a Configuration
    }

    @Produces
    @DefaultBean
    public Reporter reporter(){
        // create a Reporter
    }
}
----

The idea is that the extension auto-configures things for the user, eliminating a lot of boilerplate - we can just `@Inject` a `Tracer` wherever it is needed.
Now imagine that in our application we would like to utilize the configured `Tracer`, but we need to customize it a little, for example by providing a custom `Reporter`.
The only thing that would be needed in our application would be something like the following:


[source,java]
----
@Dependent
public class CustomTracerConfiguration {

    @Produces
    public Reporter reporter(){
        // create a custom Reporter
    }
}
----

`@DefaultBean` allows extensions (or any other code for that matter) to provide defaults while backing off if beans of that type are supplied in any
way Quarkus supports.

[[enable_build_profile]]
=== Enabling Beans for Quarkus Build Profile

Quarkus adds a capability that CDI currently does not support which is to conditionally enable a bean when a Quarkus build time profile is enabled,
via the `@io.quarkus.arc.profile.IfBuildProfile` and `@io.quarkus.arc.profile.UnlessBuildProfile` annotations.
When used in conjunction with `@io.quarkus.arc.DefaultBean`, these annotations allow for the creation of different bean configurations for different build profiles.

Imagine for instance that an application contains a bean named `Tracer`, which needs to be do nothing when in tests or dev-mode, but works in its normal capacity for the production artifact.
An elegant way to create such beans is the following:

[source,java]
----
@Dependent
public class TracerConfiguration {

    @Produces
    @IfBuildProfile("prod")
    public Tracer realTracer(Reporter reporter, Configuration configuration) {
        return new RealTracer(reporter, configuration);
    }

    @Produces
    @DefaultBean
    public Tracer noopTracer() {
        return new NoopTracer();
    }
}
----

If instead, it is required that the `Tracer` bean also works in dev-mode and only default to doing nothing for tests, then `@UnlessBuildProfile` would be ideal. The code would look like:

[source,java]
----
@Dependent
public class TracerConfiguration {

    @Produces
    @UnlessBuildProfile("test") // this will be enabled for both prod and dev build time profiles
    public Tracer realTracer(Reporter reporter, Configuration configuration) {
        return new RealTracer(reporter, configuration);
    }

    @Produces
    @DefaultBean
    public Tracer noopTracer() {
        return new NoopTracer();
    }
}
----

NOTE: The runtime profile has absolutely no effect on the bean resolution using `@IfBuildProfile` and `@UnlessBuildProfile`.

[[enable_build_properties]]
=== Enabling Beans for Quarkus Build Properties

Quarkus adds a capability that CDI currently does not support which is to conditionally enable a bean when a Quarkus build time property has/has not a specific value,
via the `@io.quarkus.arc.properties.IfBuildProperty` and `@io.quarkus.arc.properties.UnlessBuildProperty` annotations.
When used in conjunction with `@io.quarkus.arc.DefaultBean`, this annotation allow for the creation of different bean configurations for different build properties.

The scenario we mentioned above with `Tracer` could also be implemented in the following way:

[source,java]
----
@Dependent
public class TracerConfiguration {

    @Produces
    @IfBuildProperty(name = "some.tracer.enabled", stringValue = "true")
    public Tracer realTracer(Reporter reporter, Configuration configuration) {
        return new RealTracer(reporter, configuration);
    }

    @Produces
    @DefaultBean
    public Tracer noopTracer() {
        return new NoopTracer();
    }
}
----

TIP: `@IfBuildProperty` and `@UnlessBuildProperty` are repeatable annotations, i.e. a bean will only be enabled if **all** of the conditions defined by these annotations are satisfied.

If instead, it is required that the `RealTracer` bean is only used if the `some.tracer.enabled` property is not `false`, then `@UnlessBuildProperty` would be ideal. The code would look like:

[source,java]
----
@Dependent
public class TracerConfiguration {

    @Produces
    @UnlessBuildProperty(name = "some.tracer.enabled", stringValue = "false")
    public Tracer realTracer(Reporter reporter, Configuration configuration) {
        return new RealTracer(reporter, configuration);
    }

    @Produces
    @DefaultBean
    public Tracer noopTracer() {
        return new NoopTracer();
    }
}
----

NOTE: Properties set at runtime have absolutely no effect on the bean resolution using `@IfBuildProperty`.

=== Declaring Selected Alternatives

In CDI, an alternative bean may be selected either globally for an application by means of `@Priority`, or for a bean archive using a `beans.xml` descriptor.
Quarkus has a simplified bean discovery and the content of `beans.xml` is ignored.

The disadvantage of `@javax.annotation.Priority` is that it has `@Target({ TYPE, PARAMETER })` and so it cannot be used for producer methods and fields.
This problem should be fixed in Common Annotations 2.1.
Users are encouraged to use `@io.quarkus.arc.Priority` instead, until Quarkus upgrades to this version of `jakarta.annotation-api`.

However, it is also possible to select alternatives for an application using the unified configuration.
The `quarkus.arc.selected-alternatives` property accepts a list of string values that are used to match alternative beans.
If any value matches then the priority of `Integer#MAX_VALUE` is used for the relevant bean.
The priority declared via `@Priority` or `@AlternativePriority` is overridden.

.Value Examples
|===
|Value|Description
|`org.acme.Foo`| Match the fully qualified name of the bean class or the bean class of the bean that declares the producer
|`org.acme.*`| Match beans where the package of the bean class is `org.acme`
|`org.acme.**`| Match beans where the package of the bean class starts with `org.acme` 
|`Bar`| Match the simple name of the bean class or the bean class of the bean that declares the producer
|===

.Example application.properties
[source,properties]
----
quarkus.arc.selected-alternatives=org.acme.Foo,org.acme.*,Bar
----

=== Simplified Producer Method Declaration

In CDI, a producer method must be always annotated with `@Produces`.
 
[source,java]
----
class Producers {

  @Inject
  @ConfigProperty(name = "cool")
  String coolProperty;

  @Produces
  @ApplicationScoped
  MyService produceService() {
    return new MyService(coolProperty);
  }
}
----

In Quarkus, you can skip the `@Produces` annotation completely if the producer method is annotated with a scope annotation, a stereotype or a qualifier.

[source,java]
----
class Producers {

  @ConfigProperty(name = "cool")
  String coolProperty;

  @ApplicationScoped
  MyService produceService() {
    return new MyService(coolProperty);
  }
}
----

=== Interception of Static Methods 

The Interceptors specification is clear that _around-invoke_ methods must not be declared static.
However, this restriction was driven mostly by technical limitations.
And since Quarkus is a build-time oriented stack that allows for additional class transformations, those limitations don't apply anymore. 
It's possible to annotate a non-private static method with an interceptor binding:

[source,java]
----
class Services {

  @Logged <1>
  static BigDecimal computePrice(long amount) { <2>
    BigDecimal price;
    // Perform computations...
    return price;
  }
}
----
<1> `Logged` is an interceptor binding.
<2> Each method invocation is intercepted if there is an interceptor associated with `Logged`. 

==== Limitations

* Only *method-level bindings* are considered for backward compatibility reasons (otherwise static methods of bean classes that declare class-level bindings would be suddenly intercepted)
* Private static methods are never intercepted
* `InvocationContext#getTarget()` returns `null` for obvious reasons; therefore not all existing interceptors may behave correctly when intercepting static methods
+
NOTE: Interceptors can use `InvocationContext.getMethod()` to detect static methods and adjust the behavior accordingly.

=== Ability to handle 'final' classes and methods

In normal CDI, classes that are marked as `final` and / or have `final` methods are not eligible for proxy creation,
which in turn means that interceptors and normal scoped beans don't work properly.
This situation is very common when trying to use CDI with alternative JVM languages like Kotlin where classes and methods are `final` by default.

Quarkus however, can overcome these limitations when `quarkus.arc.transform-unproxyable-classes` is set to `true` (which is the default value).

=== Container-managed Concurrency

There is no standard concurrency control mechanism for CDI beans.
Nevertheless, a bean instance can be shared and accessed concurrently from multiple threads.
In that case it should be thread-safe.
You can use standard Java constructs (`volatile`, `synchronized`, `ReadWriteLock`, etc.) or let the container control the concurrent access. 
Quarkus provides `@io.quarkus.arc.Lock` and a built-in interceptor for this interceptor binding.
Each interceptor instance associated with a contextual instance of an intercepted bean holds a separate `ReadWriteLock` with non-fair ordering policy.

TIP: `io.quarkus.arc.Lock` is a regular interceptor binding and as such can be used for any bean with any scope. However, it is especially useful for "shared" scopes, e.g. `@Singleton` and `@ApplicationScoped`.

.Container-managed Concurrency Example
[source,java]
----
import io.quarkus.arc.Lock;

@Lock <1>
@ApplicationScoped
class SharedService {

  void addAmount(BigDecimal amount) {
    // ...changes some internal state of the bean
  }
  
  @Lock(value = Lock.Type.READ, time = 1, unit = TimeUnit.SECONDS) <2> <3>
  BigDecimal getAmount() {
    // ...it is safe to read the value concurrently
  }
}
----
<1> `@Lock` (which maps to `@Lock(Lock.Type.WRITE)`) declared on the class instructs the container to lock the bean instance for any invocation of any business method, i.e. the client has "exclusive access" and no concurrent invocations will be allowed.
<2> `@Lock(Lock.Type.READ)` overrides the value specified at class level. It means that any number of clients can invoke the method concurrently, unless the bean instance is locked by `@Lock(Lock.Type.WRITE)`. 
<3> You can also specify the "wait time". If it's not possible to acquire the lock in the given time a `LockException` is thrown.

=== Repeatable interceptor bindings

Quarkus has limited support for `@Repeatable` interceptor binding annotations.

When binding an interceptor to a component, you can declare multiple `@Repeatable` annotations on methods.
Repeatable interceptor bindings declared on classes and stereotypes are not supported, because there are some open questions around interactions with the Interceptors specification.
This might be added in the future.

As an example, suppose we have an interceptor that clears a cache.
The corresponding interceptor binding would be called `@CacheInvalidateAll` and would be declared as `@Repeatable`.
If we wanted to clear two caches at the same time, we would add `@CacheInvalidateAll` twice:

[source,java]
----
@ApplicationScoped
class CachingService {
  @CacheInvalidateAll(cacheName = "foo")
  @CacheInvalidateAll(cacheName = "bar")
  void heavyComputation() {
    // ...
    // some computation that updates a lot of data
    // and requires 2 caches to be invalidated
    // ...
  }
}
----

This is how interceptors are used.
What about creating an interceptor?

When declaring interceptor bindings of an interceptor, you can add multiple `@Repeatable` annotations to the interceptor class as usual.
This is useless when the annotation members are `@Nonbinding`, as would be the case for the `@Cached` annotation, but is important otherwise.

For example, suppose we have an interceptor that can automatically log method invocations to certain targets.
The interceptor binding annotation `@Logged` would have a member called `target`, which specifies where to store the log.
Our implementation could be restricted to console logging and file logging:

[source,java]
----
@Interceptor
@Logged(target = "console")
@Logged(target = "file")
class NaiveLoggingInterceptor {
  // ...
}
----

Other interceptors could be provided to log method invocations to different targets.

=== Caching the Result of Programmatic Lookup

In certain situations, it is practical to obtain a bean instance programmatically via an injected `javax.enterprise.inject.Instance` and `Instance.get()`.
However, according to the specification the `get()` method must identify the matching bean and obtain a contextual reference.
As a consequence, a new instance of a  `@Dependent` bean is returned from each invocation of `get()`.
Moreover, this instance is a dependent object of the injected `Instance`.
This behavior is well-defined but it may lead to unexpected errors and memory leaks.
Therefore, Quarkus comes with the `io.quarkus.arc.WithCaching` annotation.
An injected `Instance` annotated with this annotation will cache the result of the `Instance#get()` operation.
The result is computed on the first call and the same value is returned for all subsequent calls, even for `@Dependent` beans.

[source,java]
----
class Producer {
     
  AtomicLong nextLong = new AtomicLong();
  AtomicInteger nextInt = new AtomicInteger();
 
   @Dependent
   @Produces 
   Integer produceInt() {
     return nextInt.incrementAndGet();
   }
     
   @Dependent
   @Produces 
   Long produceLong() {
     return nextLong.incrementAndGet();
   }
}

class Consumer {
  
  @Inject
  Instance<Long> longInstance;

  @Inject
  @WithCaching
  Instance<Integer> intInstance;
     
  // this method should always return true
  // Producer#produceInt() is only called once
  boolean pingInt() {
    return intInstance.get().equals(intInstance.get());
  }

  // this method should always return false
  // Producer#produceLong() is called twice per each pingLong() invocation
  boolean pingLong() {
    return longInstance.get().equals(longInstance.get());
  }
}
----

TIP: It is also possible to clear the cached value via `io.quarkus.arc.InjectableInstance.clearCache()`. In this case, you'll need to inject the Quarkus-specific `io.quarkus.arc.InjectableInstance` instead of `javax.enterprise.inject.Instance`.

=== Declaratively Choose Beans That Can Be Obtained by Programmatic Lookup

It is sometimes useful to narrow down the set of beans that can be obtained by programmatic lookup via `javax.enterprise.inject.Instance`.
Typically, a user needs to choose the appropriate implementation of an interface based on a runtime configuration property.

Imagine that we have two beans implementing the interface `org.acme.Service`.
You can't inject the `org.acme.Service` directly unless your implementations declare a CDI qualifier.
However, you can inject the `Instance<Service>` instead, then iterate over all implementations and choose the correct one manually.
Alternatively, you can use the `@LookupIfProperty` and `@LookupUnlessProperty` annotations.
`@LookupIfProperty` indicates that a bean should only be obtained if a runtime configuration property matches the provided value.
`@LookupUnlessProperty`, on the other hand, indicates that a bean should only be obtained if a runtime configuration property does not match the provided value.

.`@LookupIfProperty` Example
[source,java]
----
 interface Service {
    String name();
 }
 
 @LookupIfProperty(name = "service.foo.enabled", stringValue = "true")
 @ApplicationScoped
 class ServiceFoo implements Service {
 
    public String name() {
       return "foo";
    }
 }
 
 @ApplicationScoped
 class ServiceBar implements Service {
 
    public String name() {
       return "bar";
    }
 }
 
 @ApplicationScoped
 class Client {
 
    @Inject
    Instance<Service> service;
    
    void printServiceName() {
       // This will print "bar" if the property "service.foo.enabled" is NOT set to "true"
       // If "service.foo.enabled" is set to "true" then service.get() would result in an AmbiguousResolutionException
       System.out.println(service.get().name());
    }
 }
----

=== Injecting Multiple Bean Instances Intuitively

In CDI, it's possible to inject multiple bean instances (aka contextual references) via the `javax.enterprise.inject.Instance` which implements `java.lang.Iterable`.
However, it's not exactly intuitive.
Therefore, a new way was introduced in Quarkus - you can inject a `java.util.List` annotated with the `io.quarkus.arc.All` qualifier.
The type of elements in the list is used as the required type when performing the lookup.

[source,java]
----
@ApplicationScoped
public class Processor {

     @Inject
     @All
     List<Service> services; <1> <2>
}
----
<1> The injected instance is an _immutable list_ of the contextual references of the _disambiguated_ beans.
<2> For this injection point the required type is `Service` and no additional qualifiers are declared.

TIP: By default, the list of beans is sorted by priority as defined by `io.quarkus.arc.InjectableBean#getPriority()`. Higher priority goes first. In general, the `@javax.annotation.Priority` and `@io.quarkus.arc.Priority` annotations can be used to assign the priority to a class bean, producer method or producer field.

If an injection point declares no other qualifier than `@All` then `@Any` is used, i.e. the behavior is equivalent to `@Inject @Any Instance<Service>`.

You can also inject a list of bean instances wrapped in `io.quarkus.arc.InstanceHandle`.
This can be useful if you need to inspect the related bean metadata.

[source,java]
----
@ApplicationScoped
public class Processor {

     @Inject
     @All
     List<InstanceHandle<Service>> services;
     
     public void doSomething() {
       for (InstanceHandle<Service> handle : services) {
         if (handle.getBean().getScope().equals(Dependent.class)) {
           handle.get().process();
           break;
         }
       }
     }
}
----

=== Ignoring Class-Level Interceptor Bindings for Methods and Constructors

If a managed bean declares interceptor binding annotations on the class level, the corresponding `@AroundInvoke` interceptors will apply to all business methods.
Similarly, the corresponding `@AroundConstruct` interceptors will apply to the bean constructor.

For example, suppose we have a logging interceptor with the `@Logged` binding annotation and a tracing interceptor with the `@Traced` binding annotation:

[source, java]
----
@ApplicationScoped
@Logged
public class MyService {
    public void doSomething() {
        ...
    }

    @Traced
    public void doSomethingElse() {
        ...
    }
}
----

In this example, both `doSomething` and `doSomethingElse` will be intercepted by the hypothetical logging interceptor.
Additionally, the `doSomethingElse` method will be intercepted by the hypothetical tracing interceptor.

Now, if that `@Traced` interceptor also performed all the necessary logging, we'd like to skip the `@Logged` interceptor for this method, but keep it for all other methods.
To achieve that, you can annotate the method with `@NoClassInterceptors`:

[source, java]
----
@Traced
@NoClassInterceptors
public void doSomethingElse() {
    ...
}
----

The `@NoClassInterceptors` annotation may be put on methods and constructors and means that all class-level interceptors are ignored for these methods and constructors.
In other words, if a method/constructor is annotated `@NoClassInterceptors`, then the only interceptors that will apply to this method/constructor are interceptors declared directly on the method/constructor.

This annotation affects only business method interceptors (`@AroundInvoke`) and constructor lifecycle callback interceptors (`@AroundConstruct`).

[[build_time_apis]]
== Build Time Extensions

Quarkus incorporates build-time optimizations in order to provide instant startup and low memory footprint.
The downside of this approach is that CDI Portable Extensions cannot be supported.
Nevertheless, most of the functionality can be achieved using Quarkus xref:writing-extensions.adoc[extensions].
See the xref:cdi-integration.adoc[integration guide] for more information.

== Development Mode

In the development mode, two special endpoints are registered automatically to provide some basic debug info in the JSON format:

* HTTP GET `/q/arc` - returns the summary; number of beans, config properties, etc.
* HTTP GET `/q/arc/beans` - returns the list of all beans
** You can use query params to filter the output:
*** `scope` - include beans with scope that ends with the given value, i.e. `http://localhost:8080/q/arc/beans?scope=ApplicationScoped`
*** `beanClass` - include beans with bean class that starts with the given value, i.e. `http://localhost:8080/q/arc/beans?beanClass=org.acme.Foo`
*** `kind` - include beans of the specified kind (`CLASS`, `PRODUCER_FIELD`, `PRODUCER_METHOD`, `INTERCEPTOR` or `SYNTHETIC`), i.e. `http://localhost:8080/q/arc/beans?kind=PRODUCER_METHOD`
* HTTP GET `/q/arc/removed-beans` - returns the list of unused beans removed during build
* HTTP GET `/q/arc/observers` - returns the list of all observer methods

NOTE: These endpoints are only available in the development mode, i.e. when you run your application via `mvn quarkus:dev` (or `./gradlew quarkusDev`).


[[arc-configuration-reference]]
== ArC Configuration Reference

include::{generated-dir}/config/quarkus-arc.adoc[leveloffset=+1, opts=optional]
