Introduction

This tutorial introduces the Fn Java FDK (Function Development Kit). If you haven't completed the Introduction to Fn tutorial you should head over there before you proceed.

This tutorial takes you through the Fn developer experience for building Java functions. It shows how easy it is to build, deploy and test functions written in Java.

As you make your way throught this tutorial, look out for this icon. Whenever you see it, it's time for you to perform an action.

Getting Started

Let's start by creating a new function. In a terminal type the following:

cd ~

mkdir javafn

cd javafn

fn init --runtime java8

The output will be:

Runtime: java
function boilerplate generated.
func.yaml created

fn init creates an simple function with a bit of boilerplate to get you started. The --runtime option is used to indicate that the function we're going to develop will be written in Java. A number of other runtimes are also supported.

The directory structure that the init command created is:

.
├── func.yaml
├── pom.xml
└── src
    ├── main
    │   └── java
    │       └── com
    │           └── example
    │               └── fn
    │                   └── HelloFunction.java
    └── test
        └── java
            └── com
                └── example
                    └── fn
                        └── HelloFunctionTest.java

11 directories, 4 files

As usual, the init command has created a func.yaml file for your function but in the case of Java it also creates a Maven pom.xml file as well as a function class and function test class.

Take a look at the contents of the generated func.yaml file.

cat func.yaml

name: javafn
version: 0.0.1
runtime: java
cmd: com.example.fn.HelloFunction::handleRequest

In the case of a Java function, the cmd property is set to the fully qualified name of the function class and the method that should be invoked when your javafn function is called.

The Java function init also generates a Maven pom.xml file to build and test your function. The pom includes the Fn Java FDK runtime and test libraries your function needs.

Running your Function

Let's build and run the generated function. We're working locally and won't be pushing our function images to a Docker registry like Docker Hub. So before we build let's set FN_REGISTRY to a local-only registry username like fndemouser. In your terminal window type:

export FN_REGISTRY=fndemouser

Now we're ready to run. Depending on whether this is your first time developing a Java function you may or may not see Docker images being pulled from Docker Hub. Once the necessary base images are downloaded subsequent operations will be faster.

As the function is built using Maven you may also see a number of Java packages being downloaded. This is also expected the first time you run a function and trigger a build.

fn run

Here's what the abbreviated output will look like:

Building image fndemouser/javafn:0.0.1
Sending build context to Docker daemon  28.67kB
Step 1/11 : FROM fnproject/fn-java-fdk-build:latest as build-stage
latest: Pulling from fnproject/fn-java-fdk-build
...
Step 2/11 : WORKDIR /function
 ---> 8ed38772a9e4
Removing intermediate container 9c3957272448
Step 3/11 : ENV MAVEN_OPTS -Dhttp.proxyHost= -Dhttp.proxyPort= -Dhttps.proxyHost= -Dhttps.proxyPort= -Dhttp.nonProxyHosts= -Dmaven.repo.local=/usr/share/maven/ref/repository
 ---> Running in 7a2e1ec6d8a5
 ---> 345e102442d0
Removing intermediate container 7a2e1ec6d8a5
Step 4/11 : ADD pom.xml /function/pom.xml
 ---> 7bd708b005e9
Step 5/11 : RUN mvn package dependency:copy-dependencies -DincludeScope=runtime -DskipTests=true -Dmdep.prependGroupId=true -DoutputDirectory=target --fail-never
 ---> Running in 51427fd1c021
[INFO] Scanning for projects...
Downloading: https://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-deploy-plugin/2.7/maven-deploy-plugin-2.7.pom
Downloaded: https://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-deploy-plugin/2.7/maven-deploy-plugin-2.7.pom (5.6 kB at 8.4 kB/s)
Downloading: https://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-deploy-plugin/2.7/maven-deploy-plugin-2.7.jar
Downloaded: https://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-deploy-plugin/2.7/maven-deploy-plugin-2.7.jar (27 kB at 188 kB/s)
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building hello 1.0.0
[INFO] ------------------------------------------------------------------------
...
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 7.853 s
[INFO] Finished at: 2017-09-20T13:50:55Z
[INFO] Final Memory: 19M/121M
[INFO] ------------------------------------------------------------------------
 ---> c010a22244a1
Removing intermediate container 51427fd1c021
Step 6/11 : ADD src /function/src
 ---> e9dd4ad1fb0c
Step 7/11 : RUN mvn package
 ---> Running in 74da2bfc5f1b
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building hello 1.0.0
[INFO] ------------------------------------------------------------------------
...
-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running com.example.fn.HelloFunctionTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.371 sec

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

[INFO]
[INFO] --- maven-jar-plugin:2.4:jar (default-jar) @ hello ---
[INFO] Building jar: /function/target/hello-1.0.0.jar
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
...
Removing intermediate container 74da2bfc5f1b
Step 8/11 : FROM fnproject/fn-java-fdk:latest
 ---> 3518e302e29e
Step 9/11 : WORKDIR /function
 ---> Using cache
 ---> 2d31a347b567
Step 10/11 : COPY --from=build-stage /function/target/*.jar /function/app/
 ---> 28f86279daf1
Step 11/11 : CMD com.example.fn.HelloFunction::handleRequest
 ---> Running in 12dd9351221a
 ---> 7617229106a0
Removing intermediate container 12dd9351221a
Successfully built 7617229106a0
Successfully tagged fndemouser/javafn:0.0.1
Hello, world!

In the output you can see Maven compiling the code and running the test, the function packaged into a container, and then run locally to produce the output "Hello, world!".

Let's try one more thing and pipe some input into the function. In your terminal type:

echo -n "Bob" | fn run

Hello, Bob!

Instead of "Hello, world!" the function has read the input string "Bob" from standard input and returned "Hello, Bob!".

Exploring the Code

We've generated, compiled, and run the Java function so let's take a look at the code in Eclipse.

Open Eclipse using the desktop icon

Import the Maven project

Select "File>Import"

Choose "Existing Maven Project"

Browse to select the Maven project directory

Select javafn in Home

Press Finish to complete the import

Switch to Java Perspective

We're working with Java so let's switch to the Java perspective.

Select "Window>Perspective>Open Perspective>Java"

The Generated Function

Expand the hello project in the Eclipse Package Explorer window and open com.example.fn.HelloFunction

Below is the generated com.example.fn.HelloFunction class. As you can see the function is just a method on a POJO that takes a string value and returns another string value, but the Java FDK also supports binding input parameters to streams, primitive types, byte arrays and Java POJOs unmarshalled from JSON. Functions can also be static or instance methods.

package com.example.fn;

public class HelloFunction {

    public String handleRequest(String input) {
        String name = (input == null || input.isEmpty()) ? "world"  : input;

        return "Hello, " + name + "!";
    }

}

This function returns the string "Hello, world!" unless an input string is provided in which case it returns "Hello, <input string>!". We saw this previously when we piped "Bob" into the function. Notice that the Java FDK reads from standard input and automatically puts the content into the string passed to the function. This greatly simplifies the function code.

Testing with JUnit

fn init also generated a JUnit test for the function which uses the Java FDK's function test framework. With this framework you can setup test fixtures with various function input values and verify the results.

The generated test confirms that when no input is provided the function returns "Hello, world!".

package com.example.fn;

import com.fnproject.fn.testing.*;
import org.junit.*;

import static org.junit.Assert.*;

public class HelloFunctionTest {

    @Rule
    public final FnTestingRule testing = FnTestingRule.createDefault();

    @Test
    public void shouldReturnGreeting() {
        testing.givenEvent().enqueue();
        testing.thenRun(HelloFunction.class, "handleRequest");

        FnResult result = testing.getOnlyResult();
        assertEquals("Hello, world!", result.getBodyAsString());
    }

}

Let's run the generated test.

Expand src/test/java under the hello project in the Eclipse Package Explorer window and open com.example.fn.HelloFunctionTest. Right click on the file name and choose "Run As>Junit Test"

The JUnit view should open and be green with one successful test.

Let's add a test that confirms that when an input string like "Bob" is provided we get the expected result.

Add the following method to HelloFunctionTest:

    @Test
    public void shouldReturnWithInput() {
        testing.givenEvent().withBody("Bob").enqueue();
        testing.thenRun(HelloFunction.class, "handleRequest");

        FnResult result = testing.getOnlyResult();
        assertEquals("Hello, Bob!", result.getBodyAsString());
    }

You can see the withBody() method used to specify the value of the function input.

Run the HelloFunctionTest class by right clicking on the file name in the Package Explorer window and choosing "Run As>Junit Test".

You should have two passing tests.

Accepting JSON Input

Let's convert this function to use JSON for its input and output.

Replace the definition of HelloFunction with the following:

package com.example.fn;

public class HelloFunction {

    public static class Input {
        public String name;
    }

    public static class Result {
        public String salutation;
    }

    public Result handleRequest(Input input) {
        Result result = new Result();
        result.salutation = "Hello " + input.name;
        return result;
    }

}

We've created a couple of simple Pojos to bind the JSON input and output to and changed the function signature to use these Pojos. The Java FDK will automatically bind input data based on the Java arguments to the function. JSON support is built-in but input and output binding is extensible and you could (for instance) plug in marshallers for other data formats like protobuf, avro or xml.

Let's rerun the tests by once again right clicking on the HelloFunctionTest class and choosing "Run As>JUnit Test".

Oops! You should see failures in the JUnit View!

We can see in the Console view the test failure exceptions--we changed the code significantly but didn't update our tests! We really should be doing test driven development and updating the test first but at least our bad behavior has been caught. Let's update the tests to reflect our new expected results. Replace the definition of HelloFunctionTest with:

package com.example.fn;

import com.fnproject.fn.testing.*;
import org.junit.*;

import static org.junit.Assert.*;

public class HelloFunctionTest {

    @Rule
    public final FnTestingRule testing = FnTestingRule.createDefault();

    @Test
    public void shouldReturnGreeting(){
        testing.givenEvent().withBody("{\"name\":\"Bob\"}").enqueue();
        testing.thenRun(HelloFunction.class,"handleRequest");

        FnResult result = testing.getOnlyResult();
        assertEquals("{\"salutation\":\"Hello Bob\"}", result.getBodyAsString());
    }
}

In the new shouldReturnGreeting() test method we're passing in the JSON document

{
    "name": "Bob"
}

and expecting a result of

{
    "salutation": "Hello Bob"
}

If you re-run the test via "Run As>JUnit Test" we can see the test now passes.

Deploying your Java Function

Now that we have our Java function updated and passing our JUnit tests we can move onto deploying it to the Fn server. As we're running the server on the local machine we can save time by not pushing the generated image out to a remote Docker repository by using the --local option.

Return to your terminal window and in the javafn directory type:

fn deploy --local --app myapp

Deploying javafn to app: myapp at path: /javafn
Bumped to version 0.0.2
Building image fndemouser/javafn:0.0.2
...
Successfully built 406b44a45821
Successfully tagged fndemouser/javafn:0.0.2
Updating route /javafn using image fndemouser/javafn:0.0.2...

The deploy command will build and test your code to make sure it's in good working order. You can see the Maven build and test output in the console. Note the last line of the output. When deployed, a function's Docker image is associated with the route specified in the func.yaml which defaults to the containing directory name. In this case the route is /javafn.

We can use the route to invoke the function via curl and passing the JSON input as the body of the call.

curl --data '{"name": "Bob"}' http://localhost:8080/r/myapp/javafn

{"salutation":"Hello Bob"}

Success!

Improving Performance

Finally you might notice that the function call takes a few hundred milliseconds. Try calling the function three times in a row paying attention to how long it takes to complete each call:

curl --data '{"name":"Tom"}' http://localhost:8080/r/myapp/javafn

By default, fn will start a new container (and therefore a new JVM) for each invocation. This may be what you want--as each function call will run in its own isolated container and process. But you can configure the function to re-use the same container and JVM for multiple invocations, thus reducing latency. This is called a 'Hot Function'. We can turn our function into a Hot Function by changing the format on the route:

fn routes update myapp /javafn --format http

Now if we call it again the first call still takes a few hundred milliseconds to start up the container but subsequent calls are super fast. Try calling the function repeatedly now that you've made the format change:

curl --data '{"name":"Tom"}' http://localhost:8080/r/myapp/javafn

Wrapping Up

Congratulations! You've just completed an introduction to the Fn Java FDK. There's so much more in the FDK than we can cover in a brief introduction but we'll go deeper in subsequent tutorials.

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Introduction

Getting Started

Running your Function

Exploring the Code

Import the Maven project

Switch to Java Perspective

The Generated Function

Testing with JUnit

Accepting JSON Input

Deploying your Java Function

Improving Performance

Wrapping Up

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Introduction

Getting Started

Running your Function

Exploring the Code

Import the Maven project

Switch to Java Perspective

The Generated Function

Testing with JUnit

Accepting JSON Input

Deploying your Java Function

Improving Performance

Wrapping Up