Provides an easy to use builder API for collecting results from multiple threads in Java or Groovy
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README.md

Lemming

Provides an easy to use builder API for collecting results from multiple threads in Java or Groovy.

Build Status Coverage Status License Maven Central

Usage

The Lemming library will collect objects from a list of Callables. The Callables can return either a single object, or a Collection of them.

The basic usage to the Lemming library is as simple as adding Callables to the Lemming builder

Lemming<Collected> lemming = Lemming.makeLemming();
Collection<Collected> results = lemming.add(new Callable<Collected>() {
    public Collected call() throws Exception {
        return new Collected();
    }
}
).add(new Callable<Collection<Collected>>() {
    public Collection<Collected> call() throws Exception {
        return Arrays.asList(new Collected(), new Collected());
    }
}).collect();

Because Callable is a functional interface, this is even easier in Groovy

Lemming<Collected> lemming = new Lemming()
lemming = lemming.add( {
    Collected
} as Callable)
lemming = lemming.add {
    [Collected, Collected]
}
Collection<Collected> foos = lemming.collect()

By default, Lemming will swallow any exceptions thrown by any of your Callables and collect results from the succeeding ones, however, if you want to fail collection if any fail, you can specify so in the builder

void testFailAllExceptionHandling() {
    shouldFail(RuntimeException) {
        Lemming.makeLemming().add {
            throw new RuntimeException()
        }.failOnAnyException().collect()
    }
}

Custom exception handling can also be specified by providing an ExceptionStrategy. An ExceptionStrategy is a Functional Interface that just needs an to implement

void handle(Throwable e)

An example in groovy

void testClosureExceptionHandling() {
    shouldFail(FooException) {
        Lemming.makeLemming().add {
            throw new RuntimeException()
        }.setExceptionStrategy {
            throw new FooException()
        }.collect()
    }
}

When working with Callables, it is often useful to set a timeout for execution

void testDefaultTimeout() {
    def results = Lemming.makeLemming().add {
        Thread.sleep(10000L)
    }.setTimeout(10L).collect()
    assert 0 == results.size()
}

Likewise, it is useful to specify to fail the collection when any of the Callables do timeout

void testFailAllTimeout() {
    shouldFail(CancellationException) {
        Lemming.makeLemming().add {
            Thread.sleep(10000L)
        }.setTimeout(10L).failOnAnyTimeout().collect()
    }
}

Thread timeouts are handled separately from standard Exceptions, and by default are swallowed, however you may specify an ExceptionHandler for timeouts

void testCustomTimeoutStrategy() {
    shouldFail(FooException) {
        Lemming.makeLemming().add {
            Thread.sleep(10000L)
        }.setTimeout(1L).setTimeoutStrategy { exception ->
            throw new FooException()
        }.collect()
    }
}

Lemming will automatically create a fixed thread pool ExecutorService for you with a thread for each Callable, and by default, it will shut it down, however if you wish to have more control over execution, you can provide an ExecutorService (and the default will change to not automatically shut it down)

def executorService = Executors.newCachedThreadPool()
Lemming.makeLemming().add {
    Collected
}.setExecutor(executorService).collect()

If you want to control the thread pool size, Lemming allows you to specify the size, that way you don't have to spin 10,000 threads if you specify 10,000 callables

void testThreadCount() {
    List<String> results = Lemming.makeLemming().add {
        Thread.currentThread().name
    }.add {
        Thread.currentThread().name
    }.setThreadCount(1).collect()
    assert 2 == results.size()
    results.each { result ->
        assert result.endsWith('-1')
    }
}

Lemming will also allow you to override the default shutdown policy of the ExecutorService as well (whether using the auto created ExecutorService or a provided one)

Lemming.makeLemming().add {
    Collected
}.setShutdownExecutorService(true).collect()

It's always useful to name your threads so while troubleshooting, you can more easily identify what the process is. Lemming allows you to specify a thread name format when using it's default ExecutorService. (Note: this is ignored when you provide an ExecutorService)

void testThreadName() {
    List<String> results = Lemming.makeLemming().add {
        Thread.currentThread().name
    }.setThreadNameFormat('TestFormat-%s').collect()
    assert results.first().startsWith('TestFormat-')
}

Advanced use cases can also set the thread priority, or if the threads will be daemon threads

void testThreadPriority() {
    List<Integer> results = Lemming.makeLemming().add {
        Thread.currentThread().priority
    }.setPriority(1).collect()
    assert 1 == results.first()
}
void testThreadDaemon() {
    List<Boolean> results = Lemming.makeLemming().add {
        Thread.currentThread().isDaemon()
    }.setDaemon(true).collect()
    assert results.first()
}

If you need to control thread creation, but are not concerned with the ExecutorService, a ThreadFactory may be provided

void testBackingThreadFactory() {
    List<String> results = Lemming.makeLemming().add {
        Thread.currentThread().name
    }.setThreadFactory { runnable ->
        def thread = new Thread(runnable)
        thread.setName('CustomThreadFactory')
        thread
    }.collect()
    assert 'CustomThreadFactory' == results.first()
}

License

Copyright 2015 Adam J. Weigold.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.