# Java

This document describes specific Java build and deploy configurations.

> **NOTE:**  Java runtime is in tech-preview.

#### In this document

- [Function and handler](#function-and-handler)
- [Build](#build)
- [Dockerfile](#dockerfile)

## Function and handler

```java
import io.nuclio.Context;
import io.nuclio.Event;
import io.nuclio.EventHandler;
import io.nuclio.Response;

public class EmptyHandler implements EventHandler {

    @Override
    public Response handleEvent(Context context, Event event) {
       return new Response().setBody("");
    }
}
```

The `handler` field must simply contain the class name. In the example above, the `handler` is `EmptyHandler`.

## Build

When instructed to build the user's handler (to create a user handler JAR), the Java runtime will generate a Gradle build script from the following template:
```
plugins {
  id 'com.github.johnrengelman.shadow' version '5.2.0'
  id 'java'
}

repositories {
    {{ range .Repositories }}
    {{ . }}
    {{ end }}
}

dependencies {
    {{ range .Dependencies }}
    compile group: '{{.Group}}', name: '{{.Name}}', version: '{{.Version}}'
    {{ end }}

    compile files('./nuclio-sdk-java-1.1.0.jar')
}

shadowJar {
   baseName = 'user-handler'
   classifier = null  // Don't append "all" to jar name
}

task userHandler(dependsOn: shadowJar)
```

The shim layer JAR is contained within the `onbuild` image, and an uber JAR is created from the user's JAR and the shim layer JAR. All dependencies (for example, `com.github.johnrengelman.shadow`) are contained within the build cache, so no internet access is required by the basic build process.

### Dependencies

The Java runtime will format `spec.build.dependencies` into the Gradle build script. For example, the following **function.yaml** section
```yaml
spec:
  build:
    dependencies:
    - "group: com.fasterxml.jackson.core, name: jackson-databind, version: 2.9.0"
    - "group: com.fasterxml.jackson.core, name: jackson-core, version: 2.9.0"
    - "group: com.fasterxml.jackson.core, name: jackson-annotations, version: 2.9.0"
```
will populate the Gradle build script as follows:
```
dependencies {
    compile group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.9.0'
    compile group: 'com.fasterxml.jackson.core', name: 'jackson-core', version: '2.9.0'
    compile group: 'com.fasterxml.jackson.core', name: 'jackson-annotations', version: '2.9.0'
}
```

### Runtime attributes

By providing the `repositories` runtime attribute, you can override the `repositories` section in the **build.gradle**. When this field is empty, `mavenCentral()` is used. For example, the following **function.yaml** section
```yaml
spec:
  build:
    runtimeAttributes:
      repositories:
      - mavenCentral()
      - jcenter()
```
will populate the Gradle build script as follows:
```
repositories {
    mavenCentral()
    jcenter()
}
```

### Custom Gradle script

Providing a **build.gradle** file inside the function directory or archive overrides the script generation.

## Dockerfile

See [Deploying Functions from a Dockerfile](../../../tasks/deploy-functions-from-dockerfile.md).

```
ARG NUCLIO_LABEL=0.5.6
ARG NUCLIO_ARCH=amd64
ARG NUCLIO_BASE_IMAGE=openjdk:11-jre-slim
ARG NUCLIO_ONBUILD_IMAGE=nuclio/handler-builder-java-onbuild:${NUCLIO_LABEL}-${NUCLIO_ARCH}

# Supplies processor, handler.jar
FROM ${NUCLIO_ONBUILD_IMAGE} as builder

# Supplies uhttpc, used for healthcheck
FROM nuclio/uhttpc:0.0.1-amd64 as uhttpc

# From the base image
FROM ${NUCLIO_BASE_IMAGE}

# Copy required objects from the suppliers
COPY --from=builder /home/gradle/bin/processor /usr/local/bin/processor
COPY --from=builder /home/gradle/src/wrapper/build/libs/nuclio-java-wrapper.jar /opt/nuclio/nuclio-java-wrapper.jar
COPY --from=uhttpc /home/nuclio/bin/uhttpc /usr/local/bin/uhttpc

# Readiness probe
HEALTHCHECK --interval=1s --timeout=3s CMD /usr/local/bin/uhttpc --url http://127.0.0.1:8082/ready || exit 1

# Run processor with configuration and platform configuration
CMD [ "processor" ]
```