Sunday, January 10, 2016

Spring Techniques: Feature toggles for controller request handler methods

Maria Gomez, a favorite colleague, asked a wonderful question, "How can I have feature toggles on Spring MVC controller request handler methods?" Existing Java feature toggle libraries focus on toggling individual beans, or using if/else logic inside methods, and don't work at the method level.

Given a trivial example toggle:

public @interface Enabled {
    boolean value();

I'd like my controller to work like this:

public class HelloWorldController {
    public static final String PATH = "/hello-world";

    private static final String texanTemplate = "Howdy, %s!";
    private static final String russianTemplate = "Привет, %s!";
    private final AtomicLong counter = new AtomicLong();

    @RequestMapping(value = "/{name}", method = GET)
    public Greeting sayHowdy(@PathVariable("name") final String name) {
        return new Greeting(counter.incrementAndGet(),
                format(texanTemplate, name));

    @RequestMapping(value = "/{name}", method = GET)
    public Greeting sayPrivet(@PathVariable("name") final String name) {
        return new Greeting(counter.incrementAndGet(),
                format(russianTemplate, name));

(Greeting is a simple struct turned into JSON by Spring.)

To make the example a little more sophisticated, I'd like to use a "3rd-party library" to decide on which features to activate (think "Togglz" or "FF4J", say):

public class EnabledChecker {
    public boolean isMapped(final Enabled enabled) {
        return null == enabled || enabled.value();

Originally I investigated Spring's RequestCondition classes, thinking I could do the same as @RequestMapping(... match conditions ...). However, this is tricky! Spring uses these conditions to decide which method to invoke for each HTTP request, not when deciding which methods should be treated as the handler for a given HTTP path. Taking this route, Spring complains at wiring time of duplicate handlers for the same request path.

The right way is to control the initial wiring of request handler methods, not decide later. First extend RequestMappingHandlerMapping (what a mouthful!):

public class EnabledRequestMappingHandlerMapping
        extends RequestMappingHandlerMapping {
    private EnabledChecker checker;

    protected RequestMappingInfo getMappingForMethod(final Method method,
            final Class<?> handlerType) {
        final Enabled enabled = findAnnotation(method, Enabled.class);
        final boolean mapped = checker.isMapped(enabled);
        return mapped ? super.getMappingForMethod(method, handlerType) : null;

Note this is not directly a bean (no @Component). We need one more bit, to override the factory method that creates these handler mappings:

public class EnabledWebMvcConfigurationSupport
        extends WebMvcConfigurationSupport {
    protected RequestMappingHandlerMapping createRequestMappingHandlerMapping() {
        return new EnabledRequestMappingHandlerMapping();

And Bob's your uncle. EnabledWebMvcConfigurationSupport ensures the returned ReqeustMappingHandlerMapping is injected, and so the "3rd-party library" is available to consult.

Full code in Github.

Thursday, December 24, 2015

Looking for problematic logging with JUnit

Stefan Birkner's System Rules is one of my favorite JUnit extension libraries. I commonly use it to verify System.out and System.err, for example validating audit trail logging.

Growing tired of the same boilerplate, I rolled some simple rules into an aggregated JUnit @Rule, called NiceLoggingRule. It enforces:

  • No logging to System.err
  • No WARN or ERROR logging to System.out

A more sophisticated version would let the user decide on more than "log level" as to what is an acceptable log line, but it gives a good demonstration of writing complex JUnit rules:

public final class NiceLoggingRule
        implements TestRule {
    private static final Pattern NEWLINE = compile("\n");

    private final SystemOutRule sout = new SystemOutRule().
    private final SystemErrRule serr = new SystemErrRule().

    private final Pattern logLinePattern;
    private final Predicate<String> problematic;
    private final RuleChain delegate;

    public NiceLoggingRule(final String logLinePattern,
            final Predicate<String> problematic) {
        this.logLinePattern = compile(logLinePattern);
        this.problematic = problematic;
        delegate = outerRule(NiceLoggingStatement::new).

    public Statement apply(final Statement base,
            final Description description) {
        return delegate.apply(base, description);

    private final class NiceLoggingStatement
            extends Statement {
        private final Statement base;
        private final Description description;

        private NiceLoggingStatement(final Statement base,
                final Description description) {
            this.base = base;
            this.description = description;

        public void evaluate()
                throws Throwable {

        private void checkSystemErr(final Description description) {
            final String cleanSerr = serr.getLogWithNormalizedLineSeparator();
            final List<String> errors = NEWLINE.splitAsStream(cleanSerr).
            if (!errors.isEmpty())
                fail("Output to System.err from " + description + ":\n"
                        + cleanSerr);

        private void checkSystemOut(final Description description) {
            final String cleanSout = sout.getLogWithNormalizedLineSeparator();
            final List<LogLine> problems = NEWLINE.splitAsStream(cleanSout).
            if (!problems.isEmpty())
                                "Problems to System.out from " + description
                                        + ":\n", "")));

    private final class LogLine {
        private final String line;
        private final String level;

        private LogLine(@Nonnull final String line) {
            final Matcher match = logLinePattern.matcher(line);
            if (!match.find()) // Not match! Ignore trailing CR?NL
                        "Log line does not match expected pattern (%s): %s",
                        logLinePattern.pattern(), line));
            this.line = line;
            level ="level");

        public boolean problematic() {
            return problematic.test(level);

        public String toString() {
            return line;

For example, using it with Spring Boot's default log pattern one might write a factory helper:

public final class SpringDefaultNiceLoggingRule {
    private static final String logLevels = Stream.of(LogLevel.values()).
            filter(level -> OFF != level).

    public static NiceLoggingRule springDefaultNiceLoggingRule() {
        return new NiceLoggingRule(
                "^(?<timestamp>\\d{4,4}-\\d{2,2}-\\d{2,2} \\d{2,2}:\\d{2,2}:\\d{2,2}\\.\\d{3,3}) +(?<level>"
                        + logLevels + ") +",

    private static boolean problematic(final String level) {
        return 0 > INFO.compareTo(LogLevel.valueOf(level));

Then a simple Spring Boot unit test becomes:

@SpringApplicationConfiguration(classes = MockServletContext.class)
public final class RootControllerTest {
    public final NiceLoggingRule niceLogging = springDefaultNiceLoggingRule();

    private MockMvc mvc;

    public void setUp()
            throws Exception {
        mvc = standaloneSetup(new RootController()).build();

    public void shouldGetRoot()
            throws Exception {
                andExpect(jsonPath("$.message", equalTo("Hello, world!")));

Monday, December 21, 2015

RESTful helper script

I find this script useful working on modern RESTful services. It shows both the headers and the formatted JSON response body. The idea is that most times you provide a URL and want to see the full response. If you need extra flags for curl just add them (e.g., user/password). If you want to customize jq—say, filter for just a particular piece of the response—use a double-dash ("--") to separate curl and jq arguments:

An example with Spring Boot (plus some custom actuator endpoints). Note "jq" colorizes the output on the command line (below is plain text):

$ ~/bin/jurlq http://localhost:8081/remote-hello/health
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Strict-Transport-Security: max-age=31536000 ; includeSubDomains
X-Application-Context: remote-hello:8081
Content-Type: application/json;charset=UTF-8
Transfer-Encoding: chunked
Date: Mon, 21 Dec 2015 13:28:07 GMT

  "status": "UP",
  "cpu": {
    "status": "UP",
    "processors": 8,
    "system-loadavg": -1,
    "process-cpu-load": 0.22963892677420872,
    "process-cpu-time": "PT42.71875S",
    "system-cpu-load": -1
  "file": {
    "status": "UP",
    "usable-disk": 55486464000,
    "total-disk": 299694551040
  "java": {
    "status": "UP",
    "start-time": "2015-12-21T07:27:06.970-06:00",
    "uptime-beats": 0,
    "vm-name": "Java HotSpot(TM) 64-Bit Server VM",
    "vm-vendor": "Oracle Corporation",
    "vm-version": "25.66-b17"
  "memory": {
    "status": "UP",
    "committed-virtual-memory": 991350784,
    "free-physical-memory": 1952854016,
    "free-swap-space": 1203003392,
    "total-physical-memory": 8540618752,
    "total-swap-space": 10354229248
  "os": {
    "status": "UP",
    "arch": "amd64",
    "name": "Windows 10",
    "version": "10.0"
  "threads": {
    "status": "UP",
    "count": 22,
    "daemon-count": 20,
    "peak-count": 22,
    "started-count": 26
  "diskSpace": {
    "status": "UP",
    "free": 55486464000,
    "threshold": 10485760
  "configServer": {
    "status": "UNKNOWN",
    "error": "no property sources located"
  "hystrix": {
    "status": "UP"

UPDATE: Tried Github Gist for the source, but it does not show in my blog feed reader. Here's the script:


for arg
    case "$arg" in
    -- ) shift ; break ;;
    * ) curl_args=("${curl_args[@]}" "$arg") ; shift ;;


curl -s -D - "${curl_args[@]}" | tr -d '\r' | {
    while read line
        case "$line" in
        '' ) echo ; break ;;
        * ) echo "$line" ;;

    exec jq "${jq_args[@]}"

Sunday, December 20, 2015

Spring REST showcase

I've noodled for some time now at mini-projects showcasing Spring REST with as many bells and whistles as I could pack in before it broke. I've never reached a satisfactory conclusion, which is more a testament to my mercurial temperament than to Spring. My ideal project would include:

  • No XML, if avoidable (Lombok, I'm looking at you)
  • No properties files (thank you, Spring YAML support)
  • Annotations and code generation (thank you, Spring Boot and Lombok)
  • Good documentation (Swagger and RAML, why do you need to be so tricky?)
  • Complete REST adherence (HATEOAS, you are still an ugly child, sorry to say that)
  • Modern Java
  • No external container (Spring Boot to the rescue!)
  • Many example integration points (and this is why I stay with you Spring)
  • Maven build (sorry Gradle, I gave up makefiles so I would never again debug my build)
  • Production support (Spring actuator: genius; see OSI)
  • Cloud support
  • Full CD pipeline (Boxfuse, you may save me yet; Github and Travis, you're still the best)

Essentially I want to implement a showcase REST microservice adhering to Larry Wall's three great virtues of a programmer: Laziness, Impatience and Hubris.

Well, there's always yet another Github repo.

Updates I'll keep updating this post as I find new things to desire in a showcase project.

Saturday, November 07, 2015

Wrong, but technically right

tldr;Immutable objects should have all-final fields.

I'm reading one of those "interview questions" posts and this jumped out at me:

Question 3: Does all property of Immutable Object needs to be final? (answer) Not necessary, as stated above you can achieve same functionality by making member as non-final but private and not modifying them except in constructor. Don't provide setter method for them and if it is a mutable object, then don't ever leak any reference for that member. Remember making a reference variable final, only ensures that it will not be reassigned a different value, but you can still change individual properties of object, pointed by that reference variable. This is one of the key point, Interviewer like to hear from candidates.

This is unfortunate. Although technically true, in my experience I've found the non-final case to come up <1% of the time. It's terribly misleading for less experienced developers to see an answer like this without having the context that goes with it.

To understand why I'm trepidatious, you can't go wrong with the C2 wiki on value objects. The right pattern for immutable objects in Java goes something like:

@RequiredArgsConstructor(staticName = "valueOf")
public final class HiMomImImmutable {
    public final int i;
    public final String s;

Here I use Lombok annotations as shorthand for the equivalent Java code. If you're not familiar with Lombok, please become so—it's both a time-saver and a bug-killer in your code.

Lombok has a @Value annotation for these. However I disagree that with their choice that fields should be private and accessed with getters. To me this makes sense when consuming frameworks requiring bean-like getters, but not otherwise.

Back to the post, the author's answer is unclear: he mentions modifying final fields in the constructor. But of course, any field marked final may be assigned in the constructor—must be so assigned—if not initialized in declaration. Then if you have complex logic to initialize (itself a code smell), try this:

public SomeClass(/* args */) {
    this.someField = aStaticMethod(/* some other args */);

This encapsulates the initialization logic in a static method as the final field may be initialized only once, and not referred to before initialization.

Tuesday, October 27, 2015

Approximating tuples in Java

What is a tuple? It's an ordered collection of individually typed elements. Wikipedia has a complicated explanation that comes down to the same thing in the context of ordinary programming. The Python implementation of tuples is a good example.

There are many related concepts, such as:

Arrays and lists
Ordered but all elements are the same type
Ordered dictionaries (maps)
Ordered but all values are of the same type, and they are named
Aggregates (structs, unions)
Individually typed but named and unordered

The closest matches to tuples in current Java are "structs" (more below) and arrays or lists, each with drawbacks. For most use of tuples, arrays and lists are non-starters: elements are all of the same nominal type. There is a lot of thought around how to do this best in Java.

What do I mean by saying Java has "structs" ala "C"? The vast bulk of Java code hides fields away with private, but exposes them with getter methods (and setters when non-final). This is the "Java Bean" anti-pattern. An alternative is to simply expose fields directly:

public final class CartesianPoint {
    public final int x;
    public final int y;

    public CartesianPoint(int x, int y) {
        this.x = x;
        this.y = y;

This is almost a tuple. However:

  • CartesianPoint extends java.lang.Object but should not be an object in the OOP sense
  • Elements are accessed by name, e.g., point.x, rather than position
  • Elements are unordered—you cannot write a general function to process the first and second elements of the tuple without knowing 1) the struct type and 2) the names of the fields

But for many use cases this can be close enough, for example as a return type to simulate multiple value return. Hopefully Java 10 brings value types which resolves the java.lang.Object issue. This may even bring true tuples!

This example can be improved with Lombok:

@RequiredArgsConstructor(staticName = "valueOf")
@ToString(includeFieldNames = false)
public final class CartesianPoint {
    public final int x;
    public final int y;

Calling code would look like:

public CartesianPoint locate() {
    int x = computeX();
    int y = computeY();

    return CartesianPoint.valueOf(x, y);

Notice the visual similarity of the local variables x and y to the corresponding fields in CartesianPoint. Users of CartesianPoint would see:

public static void main(final String... args) {
    Boat boat = Boat.rowBoat();
    CartesianPoint point = boat.locate();

    out.printf("%s -> x is %d, y is %d%n", point, point.x, point.y);

$ ./float-boat 1 2
CartesianPoint(1, 2) -> x is 1, y is 2

If you want to go hog wild, the ordered and unnamed qualities of tuples can be simulated though not without significant noise:

Function<CartesianPoint, Integer> first = p -> p.x;
Function<CartesianPoint, Integer> second = p -> p.y;

out.printf("first is %d, second is %d%n",


Streaming functionally:

Function<CartesianPoint, Integer> first = p -> p.x;
Function<CartesianPoint, Integer> second = p -> p.y;
Stream.of(first, second).
    map(f -> f.apply(point)).

Monday, October 26, 2015

What's wrong with Java 8 series by Pierre-Yves Saumont

Pierre-Yves Saumont wrote a series of articles for DZone. I feel remiss for having missed them. For example, What's Wrong in Java 8, Part IV: Monads explores java.util.Optional. Do not be misled by the post titles—yes, he criticizes Java 8 for what it could have been—, but he covers functional thinking in Java with depth, skill and panache.

The full list of "What's Wrong with Java 8" articles:

  1. Currying vs Closures
  2. Functions & Primitives
  3. Streams and Parallel Streams
  4. Monads
  5. Tuples
  6. Strictness
  7. Streams again

And all his DZone articles.

Wednesday, October 21, 2015

Tracking Java 8 stream count

I had a coding problem to fail when a Java 8 stream was empty. One way is illustrated below in example1. Another approach was interesting, shown in example2

public final class Streamy {
    private static <T, E extends RuntimeException> void example1(
            final Stream<T> items,
            final Consumer<? super T> process,
            final Supplier<E> thrown)
            throws E {
        if (0 == items.
            throw thrown.get();

    private static <T, E extends RuntimeException> void example2(
            final Stream<T> items,
            final Consumer<? super T> process,
            final Supplier<E> thrown)
            throws E {
        final AtomicBoolean foundSome = new AtomicBoolean();
        try (final Stream<T> stream = items) {
                    onClose(() -> {
                        if (!foundSome.get())
                            throw thrown.get();
                    peek(__ -> foundSome.set(true)).

    public static void main(final String... args) {
        example1(Stream.of(), out::println, RuntimeException::new);
        example2(Stream.of(), out::println, RuntimeException::new);

Comparing them I find:

  • Using count() is shorter and more clear
  • Using onClose() is more expressive

I found it odd to use peek() in example1 to execute the real purpose of the code, and was happy to discover onClose() though disappointed to need try-with-resources for it to run.

It was unfortunate that the more expressive approach (peek() for side effect, forEach() for processing, onClose for post-processing check) was also harder to understand.

Sunday, October 18, 2015

Downloading sources and javadocs automatically

Thanks to Ted Wise, I taught my "modern java" build to automatically download sources and javadocs. Using maven:


Saturday, October 17, 2015

Struggling with Travis CI and Maven 3.3

I struggled with fixing Travis CI for my "labs" code. The main problem is lack of support for Maven 3.3; my POM enforcer requires it. Eventually I got something working with help from the Internet, though I'm not happy with it. It manually downloads and uses maven 3.3:

sudo: false
language: java
  - oraclejdk8
# TODO: Gross until Travis support setting maven version or upgrades to 3.3
  - wget
  - tar zxvf apache-maven-3.3.3-bin.tar.gz
  - chmod +x apache-maven-3.3.3/bin/mvn
  - export M2_HOME=$PWD/apache-maven-3.3.3
  - export PATH=$PWD/apache-maven-3.3.3/bin:${PATH}
  - hash -r
  - export M2_HOME=$PWD/apache-maven-3.3.3
  - export PATH=$PWD/apache-maven-3.3.3/bin:${PATH}
  - hash -r
script: mvn verify -Dgpg.skip=true


On another project using Travis CI I ran afoul of the 10,000 line limit for displaying build output in the web UI. Workaround, add this to the build_install section:

  - echo 'MAVEN_OPTS="-Dorg.slf4j.simpleLogger.defaultLogLevel=warn"' >~/.mavenrc

If you're using maven 3.3 or better, Karl Heinz Marbaise has a better approach with .mvn/jvm.config.

Thursday, October 15, 2015

Spring advice

Today I had an exchange with an erstwhile colleague, one of those talented few equally comfortable in C++ and Java, great with demanding clients and fellow developers. He asked me for thoughts on Spring dependency injection. Branching out a bit, I replied:

Matter of taste/opinion.

Some things I prefer:

  • Avoid setter injection if at all possible. I want my beans to be finished after DI, not changeable at runtime on accident (very nasty bug to track down) Constructor injection most preferred, followed by field
  • Use standard annotations rather than Spring ones (@Inject), though @Value is needed for injected configuration if you're not using a dedicated configuration object
  • Spring Java configuration beats XML almost all the time
  • If the program doesn't need Spring DI, I prefer using Guice or Dagger. They're simpler, everything is at compile time, and error messages are better. The Spring non-DI libraries play fine with others (e.g., spring-jdbc)
  • Avoid mixing business objects with wiring unless it makes sense. For example, JdbcTemplate should be new'ed as needed, injecting only the DataSource. Injecting the template is an anti-pattern
  • Do use Spring Boot or Dropwizard, et al, if it makes sense. Big time savers, lots of good integration prepackaged
  • For webby programs (apps, services) remember to test unit, controller and integration separately. Spring boot intro page has excellent code examples

Things get more interesting with multiple configurations and with cloud. For example:

  • Do you make separate builds for each env, or one build with multiple configurations? Latter is traditional in EE world, former much better for cloud (devops, immutable, unikernel, etc)
  • Do you pull in configuration externally, e.g., Spring Cloud Config, Netflix Archaius or Apache Zookeeper. I like this approach, but more complex and overkill for simple programs. Nearly mandatory for microservices, and strong choice when in cloud

Did I answer fairly?

Tuesday, October 13, 2015

Blog code 6

(Updated) I've published my blog code version 6 to Maven Central with javadocs. The previous version was 0.5; having a leading "0." was silly (there won't be a 1.0).

Interesting changes:

  • Added the YAML modules. This is an annotation processor which takes YAML descriptions of Java data structures, and generates immutable classes for them.
  • Added the Matching class for a pattern-matching DSL in Java. I'll improve on over time.