In order to be able to use transactions with our thread we need to understand how transactions work with spring. Transaction information in spring is stored in ThreadLocal variables. Therefore these variables are specific for an ongoing transaction on a single thread.

When it comes to an action run by a single thread the transaction gets propagated among the spring components called hierarchically.

Thus in case of a @Transactional  annotated service which spawns a thread, the transaction will not be propagated from the @Transactional service to the newly created thread. The result will be an error indicating that the transaction is missing.

Since the action that take place inside your thread, requires database access through jpa, a new transaction has to be created.

By looking at the @Transactional documentation  we can get more information on the transaction propagation types. The default propagation mode for @Transactional is REQUIRED.

Therefore by annotating a method with the @Transactional, a new transaction will be created and will be propagated to the other services called from our thread.

For example our async method can be annotated as Transactional

@Async
@Transactional
public void executeTransactionally() {
    System.out.println("Execute a transaction from the new thread");
}

The same applies for the method which will be invoked from the run function of a Runnable class. Although async is pretty simple to use, behind the scenes it wraps the call in a Runnable, which dispatched to an executor.

To sum up when it comes to work with threads and transaction in spring, it should be done with extra care. Also keep in mind that the transactions cannot be passed from thread to thread. Last but not least make sure that your @Async and @Transactional functions are public and go though the proxy that will make the necessary actions before being invoked.

Last but not least I’ve compiled a cheat sheet that lists some helpful spring & threads tips.
Sign up in the link to receive it.

Previously we started working with spring and the TaskExecutor, thus we became more familiar on how to use threads on a spring application.

However using the task executor might be cumbersome especially when we need to execute a simple action.

Spring’s Asynchronous methods come to the rescue.

Instead of messing with runnables and the TaskExecutor, you trade the control of the executor for the simplicity of the async functions.
In order to execute your function in another thread all you have to do is to annotate your functions with the @Async annotation.

Asynchronous methods come with two modes.

A fire and forget mode: a method which returns a void type.

    @Async
    @Transactional
    public void printEmployees() {

        List<Employee> employees = entityManager.createQuery("SELECT e FROM Employee e").getResultList();
        employees.stream().forEach(e->System.out.println(e.getEmail()));
    }

A results retrieval mode: a method which returns a future type.

    @Async
    @Transactional
    public CompletableFuture<List<Employee>> fetchEmployess() {
        List<Employee> employees = entityManager.createQuery("SELECT e FROM Employee e").getResultList();
        return CompletableFuture.completedFuture(employees);
    }

Pay extra attention to the fact that @Async annotations do not work if they are invoked by ‘this’. @Async behaves just like the @Transactional annotation. Therefore you need to have your async functions as public. You can find more information on the aop proxies documentation.

However using only the @Async annotation is not enough. We need to enable Spring’s asynchronous method execution capability by using the @EnableAsync annotation in one of our configuration classes.

package com.gkatzioura.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.task.TaskExecutor;
import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;

import java.util.concurrent.Executor;

/**
 * Created by gkatzioura on 4/26/17.
 */
@Configuration
@EnableAsync
public class ThreadConfig {

    @Bean
    public TaskExecutor threadPoolTaskExecutor() {

        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(4);
        executor.setMaxPoolSize(4);
        executor.setThreadNamePrefix("sgfgd");
        executor.initialize();

        return executor;
    }

}

The next question is how we declare the resources and the threads pools that the async functions will use. We can get the answer from the documentation.

By default, Spring will be searching for an associated thread pool definition: either a unique TaskExecutor bean in the context, or an Executor bean named “taskExecutor” otherwise. If neither of the two is resolvable, a SimpleAsyncTaskExecutor will be used to process async method invocations.

However in some cases we don’t want the same thread pool to run all of application’s tasks. We might want separate threads pools with different configurations backing our functions.

To achieve so we pass to the @Async annotation the name of the executor we might want to use for each function.

For example  an executor with the name ‘specificTaskExecutor’ is configured.

@Configuration
@EnableAsync
public class ThreadConfig {

    @Bean(name = "specificTaskExecutor")
    public TaskExecutor specificTaskExecutor() {

        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.initialize();
        return executor;
    }

}

Then our function should set the qualifier value to determine the target executor of a specific Executor or TaskExecutor.

@Async("specificTaskExecutor")
public void runFromAnotherThreadPool() {
    System.out.println("You function code here");
}

On the next article we will talk about transactions on threads.

You can find the sourcecode on github.

Last but not least I’ve compiled a cheat sheet that lists some helpful spring & threads tips.
Sign up in the link to receive it.

Using threads in a web application is not unusual especially when you have to develop long running tasks.

Considering spring we must pay extra attention and use the tools it already provides, instead of spawning our own threads.
We want our threads to be managed by spring and thus be able to use the other components of our application without any repercussions, and shutdown our application gracefully without any work being in progress.

Spring provides the TaskExecutor as an abstraction for dealing with executors.
The Spring’s TaskExecutor interface is identical to the java.util.concurrent.Executor interface.
There are a number of pre-built implementations of TaskExecutor included with the Spring distribution, you can find more about them from the official documentation.
By providing to your spring environment a TaskExecutor implementation you will be able to inject the TaskExecutor to your beans and have access to managed threads.

package com.gkatzioura.service;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.core.task.TaskExecutor;
import org.springframework.stereotype.Service;
import java.util.List;

/**
 * Created by gkatzioura on 4/26/17.
 */
@Service
public class AsynchronousService {

    @Autowired
    private ApplicationContext applicationContext;

    @Autowired
    private TaskExecutor taskExecutor;

    public void executeAsynchronously() {

        taskExecutor.execute(new Runnable() {
            @Override
            public void run() {
                //TODO add long running task
            }
        });
    }
}

First step is to add the TaskExecutor configuration to our spring application.

package com.gkatzioura.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.task.TaskExecutor;
import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;

import java.util.concurrent.Executor;

/**
 * Created by gkatzioura on 4/26/17.
 */
@Configuration
public class ThreadConfig {

    @Bean
    public TaskExecutor threadPoolTaskExecutor() {

        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(4);
        executor.setMaxPoolSize(4);
        executor.setThreadNamePrefix("default_task_executor_thread");
        executor.initialize();

        return executor;
    }

}

Once we have our executor setup the process is simple. We inject the executor to a spring component and then we submit Runnable classes containing the tasks to be executed.

Since our asynchronous code might as well need to interact with other components of our application and have them injected, a nice approach is to create prototype scoped runnable instances.

package com.gkatzioura;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Component;

/**
 * Created by gkatzioura on 10/18/17.
 */
@Component
@Scope("prototype")
public class MyThread implements Runnable {

    private static final Logger LOGGER = LoggerFactory.getLogger(MyThread.class);

    @Override
    public void run() {
        
        LOGGER.info("Called from thread");
    }
}

Then we are ready to inject the executor to our services and use it to execute runnable instances.

package com.gkatzioura.service;

import com.gkatzioura.MyThread;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.core.task.TaskExecutor;
import org.springframework.stereotype.Service;

import java.util.List;

/**
 * Created by gkatzioura on 4/26/17.
 */
@Service
public class AsynchronousService {

    @Autowired
    private TaskExecutor taskExecutor;

    @Autowired
    private ApplicationContext applicationContext;

    public void executeAsynchronously() {

        MyThread myThread = applicationContext.getBean(MyThread.class);
        taskExecutor.execute(myThread);
    }

}

On the next article we will bring our multit-hreaded codebase to a new level by using spring’s asynchronous functions.

You can find the sourcecode on github.

Last but not least I’ve compiled a cheat sheet that lists some helpful spring & threads tips.
Sign up in the link to receive it.