https://github.com/Netflix/Hystrix: hystrix-core
  • Netflix Open Source Development 
Copyright 2012 Netflix, Inc.
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.

/**
 * Copyright 2012 Netflix, Inc.
 * 
 * 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.
 */
package com.netflix.hystrix;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicReference;

import com.netflix.hystrix.util.Exceptions;
import rx.Observable;
import rx.functions.Action0;

import com.netflix.hystrix.exception.HystrixBadRequestException;
import com.netflix.hystrix.exception.HystrixRuntimeException;
import com.netflix.hystrix.exception.HystrixRuntimeException.FailureType;
import com.netflix.hystrix.strategy.executionhook.HystrixCommandExecutionHook;
import com.netflix.hystrix.strategy.properties.HystrixPropertiesStrategy;
import rx.functions.Func0;


Used to wrap code that will execute potentially risky functionality (typically meaning a service call over the network)
with fault and latency tolerance, statistics and performance metrics capture, circuit breaker and bulkhead functionality.
This command is essentially a blocking command but provides an Observable facade if used with observe()

Type parameters:
  • <R> – 
           the return type
@ThreadSafe
/**
 * Used to wrap code that will execute potentially risky functionality (typically meaning a service call over the network)
 * with fault and latency tolerance, statistics and performance metrics capture, circuit breaker and bulkhead functionality.
 * This command is essentially a blocking command but provides an Observable facade if used with observe()
 * 
 * @param <R>
 *            the return type
 * 
 * @ThreadSafe
 */
public abstract class HystrixCommand<R> extends AbstractCommand<R> implements HystrixExecutable<R>, HystrixInvokableInfo<R>, HystrixObservable<R> {


    
Construct a HystrixCommand with defined HystrixCommandGroupKey. 
 The HystrixCommandKey will be derived from the implementing class name. 
Params:
/**
     * Construct a {@link HystrixCommand} with defined {@link HystrixCommandGroupKey}.
     * <p>
     * The {@link HystrixCommandKey} will be derived from the implementing class name.
     * 
     * @param group
     *            {@link HystrixCommandGroupKey} used to group together multiple {@link HystrixCommand} objects.
     *            <p>
     *            The {@link HystrixCommandGroupKey} is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interact with,
     *            common business purpose etc.
     */
    protected HystrixCommand(HystrixCommandGroupKey group) {
        super(group, null, null, null, null, null, null, null, null, null, null, null);
    }


    
Construct a HystrixCommand with defined HystrixCommandGroupKey and HystrixThreadPoolKey. 
 The HystrixCommandKey will be derived from the implementing class name. 
Params:
/**
     * Construct a {@link HystrixCommand} with defined {@link HystrixCommandGroupKey} and {@link HystrixThreadPoolKey}.
     * <p>
     * The {@link HystrixCommandKey} will be derived from the implementing class name.
     *
     * @param group
     *            {@link HystrixCommandGroupKey} used to group together multiple {@link HystrixCommand} objects.
     *            <p>
     *            The {@link HystrixCommandGroupKey} is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interact with,
     *            common business purpose etc.
     * @param threadPool
     *            {@link HystrixThreadPoolKey} used to identify the thread pool in which a {@link HystrixCommand} executes.
     */
    protected HystrixCommand(HystrixCommandGroupKey group, HystrixThreadPoolKey threadPool) {
        super(group, null, threadPool, null, null, null, null, null, null, null, null, null);
    }

    
Construct a HystrixCommand with defined HystrixCommandGroupKey and thread timeout 
 The HystrixCommandKey will be derived from the implementing class name. 
Params:
  • group –  HystrixCommandGroupKey used to group together multiple HystrixCommand objects. 
     The HystrixCommandGroupKey is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interact with, common business purpose etc.
  • executionIsolationThreadTimeoutInMilliseconds –  Time in milliseconds at which point the calling thread will timeout (using Future.get) and walk away from the executing thread.
/**
     * Construct a {@link HystrixCommand} with defined {@link HystrixCommandGroupKey} and thread timeout
     * <p>
     * The {@link HystrixCommandKey} will be derived from the implementing class name.
     *
     * @param group
     *            {@link HystrixCommandGroupKey} used to group together multiple {@link HystrixCommand} objects.
     *            <p>
     *            The {@link HystrixCommandGroupKey} is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interact with,
     *            common business purpose etc.
     * @param executionIsolationThreadTimeoutInMilliseconds
     *            Time in milliseconds at which point the calling thread will timeout (using {@link Future#get}) and walk away from the executing thread.
     */
    protected HystrixCommand(HystrixCommandGroupKey group, int executionIsolationThreadTimeoutInMilliseconds) {
        super(group, null, null, null, null, HystrixCommandProperties.Setter().withExecutionTimeoutInMilliseconds(executionIsolationThreadTimeoutInMilliseconds), null, null, null, null, null, null);
    }

    
Construct a HystrixCommand with defined HystrixCommandGroupKey, HystrixThreadPoolKey, and thread timeout. 
 The HystrixCommandKey will be derived from the implementing class name. 
Params:
  • group –  HystrixCommandGroupKey used to group together multiple HystrixCommand objects. 
     The HystrixCommandGroupKey is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interact with, common business purpose etc.
  • threadPool –  HystrixThreadPool used to identify the thread pool in which a HystrixCommand executes.
  • executionIsolationThreadTimeoutInMilliseconds –  Time in milliseconds at which point the calling thread will timeout (using Future.get) and walk away from the executing thread.
/**
     * Construct a {@link HystrixCommand} with defined {@link HystrixCommandGroupKey}, {@link HystrixThreadPoolKey}, and thread timeout.
     * <p>
     * The {@link HystrixCommandKey} will be derived from the implementing class name.
     *
     * @param group
     *            {@link HystrixCommandGroupKey} used to group together multiple {@link HystrixCommand} objects.
     *            <p>
     *            The {@link HystrixCommandGroupKey} is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interact with,
     *            common business purpose etc.
     * @param threadPool
     *            {@link HystrixThreadPool} used to identify the thread pool in which a {@link HystrixCommand} executes.
     * @param executionIsolationThreadTimeoutInMilliseconds
     *            Time in milliseconds at which point the calling thread will timeout (using {@link Future#get}) and walk away from the executing thread.
     */
    protected HystrixCommand(HystrixCommandGroupKey group, HystrixThreadPoolKey threadPool, int executionIsolationThreadTimeoutInMilliseconds) {
        super(group, null, threadPool, null, null, HystrixCommandProperties.Setter().withExecutionTimeoutInMilliseconds(executionIsolationThreadTimeoutInMilliseconds), null, null, null, null, null, null);
    }

    
Construct a HystrixCommand with defined Setter that allows injecting property and strategy overrides and other optional arguments. 
 NOTE: The HystrixCommandKey is used to associate a HystrixCommand with HystrixCircuitBreaker, HystrixCommandMetrics and other objects. 
 Do not create multiple HystrixCommand implementations with the same HystrixCommandKey but different injected default properties as the first instantiated will win. 
 Properties passed in via Setter.andCommandPropertiesDefaults or Setter.andThreadPoolPropertiesDefaults are cached for the given HystrixCommandKey for the life of the JVM or until Hystrix.reset() is called. Dynamic properties allow runtime changes. Read more on the Hystrix Wiki.

Params:
  • setter – 
           Fluent interface for constructor arguments
/**
     * Construct a {@link HystrixCommand} with defined {@link Setter} that allows injecting property and strategy overrides and other optional arguments.
     * <p>
     * NOTE: The {@link HystrixCommandKey} is used to associate a {@link HystrixCommand} with {@link HystrixCircuitBreaker}, {@link HystrixCommandMetrics} and other objects.
     * <p>
     * Do not create multiple {@link HystrixCommand} implementations with the same {@link HystrixCommandKey} but different injected default properties as the first instantiated will win.
     * <p>
     * Properties passed in via {@link Setter#andCommandPropertiesDefaults} or {@link Setter#andThreadPoolPropertiesDefaults} are cached for the given {@link HystrixCommandKey} for the life of the JVM
     * or until {@link Hystrix#reset()} is called. Dynamic properties allow runtime changes. Read more on the <a href="https://github.com/Netflix/Hystrix/wiki/Configuration">Hystrix Wiki</a>.
     * 
     * @param setter
     *            Fluent interface for constructor arguments
     */
    protected HystrixCommand(Setter setter) {
        // use 'null' to specify use the default
        this(setter.groupKey, setter.commandKey, setter.threadPoolKey, null, null, setter.commandPropertiesDefaults, setter.threadPoolPropertiesDefaults, null, null, null, null, null);
    }

    
Allow constructing a HystrixCommand with injection of most aspects of its functionality. 

Some of these never have a legitimate reason for injection except in unit testing.


Most of the args will revert to a valid default if 'null' is passed in.

/**
     * Allow constructing a {@link HystrixCommand} with injection of most aspects of its functionality.
     * <p>
     * Some of these never have a legitimate reason for injection except in unit testing.
     * <p>
     * Most of the args will revert to a valid default if 'null' is passed in.
     */
    /* package for testing */HystrixCommand(HystrixCommandGroupKey group, HystrixCommandKey key, HystrixThreadPoolKey threadPoolKey, HystrixCircuitBreaker circuitBreaker, HystrixThreadPool threadPool,
            HystrixCommandProperties.Setter commandPropertiesDefaults, HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults,
            HystrixCommandMetrics metrics, TryableSemaphore fallbackSemaphore, TryableSemaphore executionSemaphore,
            HystrixPropertiesStrategy propertiesStrategy, HystrixCommandExecutionHook executionHook) {
        super(group, key, threadPoolKey, circuitBreaker, threadPool, commandPropertiesDefaults, threadPoolPropertiesDefaults, metrics, fallbackSemaphore, executionSemaphore, propertiesStrategy, executionHook);
    }

    
Fluent interface for arguments to the HystrixCommand constructor. 

The required arguments are set via the 'with' factory method and optional arguments via the 'and' chained methods.


Example:

 
 Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("GroupName"))
.andCommandKey(HystrixCommandKey.Factory.asKey("CommandName"));
 


@NotThreadSafe
/**
     * Fluent interface for arguments to the {@link HystrixCommand} constructor.
     * <p>
     * The required arguments are set via the 'with' factory method and optional arguments via the 'and' chained methods.
     * <p>
     * Example:
     * <pre> {@code
     *  Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("GroupName"))
                .andCommandKey(HystrixCommandKey.Factory.asKey("CommandName"));
     * } </pre>
     * 
     * @NotThreadSafe
     */
    final public static class Setter {

        protected final HystrixCommandGroupKey groupKey;
        protected HystrixCommandKey commandKey;
        protected HystrixThreadPoolKey threadPoolKey;
        protected HystrixCommandProperties.Setter commandPropertiesDefaults;
        protected HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults;

        
Setter factory method containing required values.


All optional arguments can be set via the chained methods.

Params:
  • groupKey –  HystrixCommandGroupKey used to group together multiple HystrixCommand objects. 
     The HystrixCommandGroupKey is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace with, common business purpose etc.
/**
         * Setter factory method containing required values.
         * <p>
         * All optional arguments can be set via the chained methods.
         * 
         * @param groupKey
         *            {@link HystrixCommandGroupKey} used to group together multiple {@link HystrixCommand} objects.
         *            <p>
         *            The {@link HystrixCommandGroupKey} is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace
         *            with,
         *            common business purpose etc.
         */
        protected Setter(HystrixCommandGroupKey groupKey) {
            this.groupKey = groupKey;
        }

        
Setter factory method with required values.


All optional arguments can be set via the chained methods.

Params:
  • groupKey –  HystrixCommandGroupKey used to group together multiple HystrixCommand objects. 
     The HystrixCommandGroupKey is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace with, common business purpose etc.
/**
         * Setter factory method with required values.
         * <p>
         * All optional arguments can be set via the chained methods.
         * 
         * @param groupKey
         *            {@link HystrixCommandGroupKey} used to group together multiple {@link HystrixCommand} objects.
         *            <p>
         *            The {@link HystrixCommandGroupKey} is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace
         *            with,
         *            common business purpose etc.
         */
        public static Setter withGroupKey(HystrixCommandGroupKey groupKey) {
            return new Setter(groupKey);
        }

        
Params:
Returns:Setter for fluent interface via method chaining
/**
         * @param commandKey
         *            {@link HystrixCommandKey} used to identify a {@link HystrixCommand} instance for statistics, circuit-breaker, properties, etc.
         *            <p>
         *            By default this will be derived from the instance class name.
         *            <p>
         *            NOTE: Every unique {@link HystrixCommandKey} will result in new instances of {@link HystrixCircuitBreaker}, {@link HystrixCommandMetrics} and {@link HystrixCommandProperties}.
         *            Thus,
         *            the number of variants should be kept to a finite and reasonable number to avoid high-memory usage or memory leaks.
         *            <p>
         *            Hundreds of keys is fine, tens of thousands is probably not.
         * @return Setter for fluent interface via method chaining
         */
        public Setter andCommandKey(HystrixCommandKey commandKey) {
            this.commandKey = commandKey;
            return this;
        }

        
Params:
Returns:Setter for fluent interface via method chaining
/**
         * @param threadPoolKey
         *            {@link HystrixThreadPoolKey} used to define which thread-pool this command should run in (when configured to run on separate threads via
         *            {@link HystrixCommandProperties#executionIsolationStrategy()}).
         *            <p>
         *            By default this is derived from the {@link HystrixCommandGroupKey} but if injected this allows multiple commands to have the same {@link HystrixCommandGroupKey} but different
         *            thread-pools.
         * @return Setter for fluent interface via method chaining
         */
        public Setter andThreadPoolKey(HystrixThreadPoolKey threadPoolKey) {
            this.threadPoolKey = threadPoolKey;
            return this;
        }

        
Optional

Params:
Returns:Setter for fluent interface via method chaining
/**
         * Optional
         * 
         * @param commandPropertiesDefaults
         *            {@link HystrixCommandProperties.Setter} with property overrides for this specific instance of {@link HystrixCommand}.
         *            <p>
         *            See the {@link HystrixPropertiesStrategy} JavaDocs for more information on properties and order of precedence.
         * @return Setter for fluent interface via method chaining
         */
        public Setter andCommandPropertiesDefaults(HystrixCommandProperties.Setter commandPropertiesDefaults) {
            this.commandPropertiesDefaults = commandPropertiesDefaults;
            return this;
        }

        
Optional

Params:
Returns:Setter for fluent interface via method chaining
/**
         * Optional
         * 
         * @param threadPoolPropertiesDefaults
         *            {@link HystrixThreadPoolProperties.Setter} with property overrides for the {@link HystrixThreadPool} used by this specific instance of {@link HystrixCommand}.
         *            <p>
         *            See the {@link HystrixPropertiesStrategy} JavaDocs for more information on properties and order of precedence.
         * @return Setter for fluent interface via method chaining
         */
        public Setter andThreadPoolPropertiesDefaults(HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults) {
            this.threadPoolPropertiesDefaults = threadPoolPropertiesDefaults;
            return this;
        }

    }

	private final AtomicReference<Thread> executionThread = new AtomicReference<Thread>();
	private final AtomicBoolean interruptOnFutureCancel = new AtomicBoolean(false);

	
Implement this method with code to be executed when execute() or queue() are invoked. 
Throws:
Returns:R response type
/**
     * Implement this method with code to be executed when {@link #execute()} or {@link #queue()} are invoked.
     * 
     * @return R response type
     * @throws Exception
     *             if command execution fails
     */
    protected abstract R run() throws Exception;

    
If execute() or queue() fails in any way then this method will be invoked to provide an opportunity to return a fallback response. 

This should do work that does not require network transport to produce.


In other words, this should be a static or cached result that can immediately be returned upon failure.

 If network traffic is wanted for fallback (such as going to MemCache) then the fallback implementation should invoke another HystrixCommand instance that protects against that network access and possibly has another level of fallback that does not involve network access. 

DEFAULT BEHAVIOR: It throws UnsupportedOperationException.

Returns:R or throw UnsupportedOperationException if not implemented
/**
     * If {@link #execute()} or {@link #queue()} fails in any way then this method will be invoked to provide an opportunity to return a fallback response.
     * <p>
     * This should do work that does not require network transport to produce.
     * <p>
     * In other words, this should be a static or cached result that can immediately be returned upon failure.
     * <p>
     * If network traffic is wanted for fallback (such as going to MemCache) then the fallback implementation should invoke another {@link HystrixCommand} instance that protects against that network
     * access and possibly has another level of fallback that does not involve network access.
     * <p>
     * DEFAULT BEHAVIOR: It throws UnsupportedOperationException.
     * 
     * @return R or throw UnsupportedOperationException if not implemented
     */
    protected R getFallback() {
        throw new UnsupportedOperationException("No fallback available.");
    }

    @Override
    final protected Observable<R> getExecutionObservable() {
        return Observable.defer(new Func0<Observable<R>>() {
            @Override
            public Observable<R> call() {
                try {
                    return Observable.just(run());
                } catch (Throwable ex) {
                    return Observable.error(ex);
                }
            }
        }).doOnSubscribe(new Action0() {
            @Override
            public void call() {
                // Save thread on which we get subscribed so that we can interrupt it later if needed
                executionThread.set(Thread.currentThread());
            }
        });
    }

    @Override
    final protected Observable<R> getFallbackObservable() {
        return Observable.defer(new Func0<Observable<R>>() {
            @Override
            public Observable<R> call() {
                try {
                    return Observable.just(getFallback());
                } catch (Throwable ex) {
                    return Observable.error(ex);
                }
            }
        });
    }

    
Used for synchronous execution of command.

Throws:
Returns:R Result of run() execution or a fallback from getFallback() if the command fails for any reason.
/**
     * Used for synchronous execution of command.
     * 
     * @return R
     *         Result of {@link #run()} execution or a fallback from {@link #getFallback()} if the command fails for any reason.
     * @throws HystrixRuntimeException
     *             if a failure occurs and a fallback cannot be retrieved
     * @throws HystrixBadRequestException
     *             if invalid arguments or state were used representing a user failure, not a system failure
     * @throws IllegalStateException
     *             if invoked more than once
     */
    public R execute() {
        try {
            return queue().get();
        } catch (Exception e) {
            throw Exceptions.sneakyThrow(decomposeException(e));
        }
    }

    
Used for asynchronous execution of command.

 This will queue up the command on the thread pool and return an Future to get the result once it completes. 
 NOTE: If configured to not run in a separate thread, this will have the same effect as execute() and will block. 

We don't throw an exception but just flip to synchronous execution so code doesn't need to change in order to switch a command from running on a separate thread to the calling thread.

Throws:
Returns:Future<R> Result of run() execution or a fallback from getFallback() if the command fails for any reason.
/**
     * Used for asynchronous execution of command.
     * <p>
     * This will queue up the command on the thread pool and return an {@link Future} to get the result once it completes.
     * <p>
     * NOTE: If configured to not run in a separate thread, this will have the same effect as {@link #execute()} and will block.
     * <p>
     * We don't throw an exception but just flip to synchronous execution so code doesn't need to change in order to switch a command from running on a separate thread to the calling thread.
     * 
     * @return {@code Future<R>} Result of {@link #run()} execution or a fallback from {@link #getFallback()} if the command fails for any reason.
     * @throws HystrixRuntimeException
     *             if a fallback does not exist
     *             <p>
     *             <ul>
     *             <li>via {@code Future.get()} in {@link ExecutionException#getCause()} if a failure occurs</li>
     *             <li>or immediately if the command can not be queued (such as short-circuited, thread-pool/semaphore rejected)</li>
     *             </ul>
     * @throws HystrixBadRequestException
     *             via {@code Future.get()} in {@link ExecutionException#getCause()} if invalid arguments or state were used representing a user failure, not a system failure
     * @throws IllegalStateException
     *             if invoked more than once
     */
    public Future<R> queue() {
        /*
         * The Future returned by Observable.toBlocking().toFuture() does not implement the
         * interruption of the execution thread when the "mayInterrupt" flag of Future.cancel(boolean) is set to true;
         * thus, to comply with the contract of Future, we must wrap around it.
         */
        final Future<R> delegate = toObservable().toBlocking().toFuture();
    	
        final Future<R> f = new Future<R>() {

            @Override
            public boolean cancel(boolean mayInterruptIfRunning) {
                if (delegate.isCancelled()) {
                    return false;
                }

                if (HystrixCommand.this.getProperties().executionIsolationThreadInterruptOnFutureCancel().get()) {
                    /*
                     * The only valid transition here is false -> true. If there are two futures, say f1 and f2, created by this command
                     * (which is super-weird, but has never been prohibited), and calls to f1.cancel(true) and to f2.cancel(false) are
                     * issued by different threads, it's unclear about what value would be used by the time mayInterruptOnCancel is checked.
                     * The most consistent way to deal with this scenario is to say that if *any* cancellation is invoked with interruption,
                     * than that interruption request cannot be taken back.
                     */
                    interruptOnFutureCancel.compareAndSet(false, mayInterruptIfRunning);
        		}

                final boolean res = delegate.cancel(interruptOnFutureCancel.get());

                if (!isExecutionComplete() && interruptOnFutureCancel.get()) {
                    final Thread t = executionThread.get();
                    if (t != null && !t.equals(Thread.currentThread())) {
                        t.interrupt();
                    }
                }

                return res;
			}

            @Override
            public boolean isCancelled() {
                return delegate.isCancelled();
			}

            @Override
            public boolean isDone() {
                return delegate.isDone();
			}

            @Override
            public R get() throws InterruptedException, ExecutionException {
                return delegate.get();
            }

            @Override
            public R get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
                return delegate.get(timeout, unit);
            }
        	
        };

        /* special handling of error states that throw immediately */
        if (f.isDone()) {
            try {
                f.get();
                return f;
            } catch (Exception e) {
                Throwable t = decomposeException(e);
                if (t instanceof HystrixBadRequestException) {
                    return f;
                } else if (t instanceof HystrixRuntimeException) {
                    HystrixRuntimeException hre = (HystrixRuntimeException) t;
                    switch (hre.getFailureType()) {
					case COMMAND_EXCEPTION:
					case TIMEOUT:
						// we don't throw these types from queue() only from queue().get() as they are execution errors
						return f;
					default:
						// these are errors we throw from queue() as they as rejection type errors
						throw hre;
					}
                } else {
                    throw Exceptions.sneakyThrow(t);
                }
            }
        }

        return f;
    }

    @Override
    protected String getFallbackMethodName() {
        return "getFallback";
    }

    @Override
    protected boolean isFallbackUserDefined() {
        Boolean containsFromMap = commandContainsFallback.get(commandKey);
        if (containsFromMap != null) {
            return containsFromMap;
        } else {
            Boolean toInsertIntoMap;
            try {
                getClass().getDeclaredMethod("getFallback");
                toInsertIntoMap = true;
            } catch (NoSuchMethodException nsme) {
                toInsertIntoMap = false;
            }
            commandContainsFallback.put(commandKey, toInsertIntoMap);
            return toInsertIntoMap;
        }
    }

    @Override
    protected boolean commandIsScalar() {
        return true;
    }
}