What is the difference between a FixedThreadPool and ThreadPoolTaskExecutor?

ThreadPoolTaskExecutor is a class from Spring Framework. On the other hand Executors::newFixedThreadPool creates a standard ThreadPoolExecutor thread pool that comes from standard Java and is available since Java 5.

From docs of ThreadPoolTaskExecutor :

JavaBean that allows for configuring a ThreadPoolExecutor in bean style (through its "corePoolSize", "maxPoolSize", "keepAliveSeconds", "queueCapacity" properties) and exposing it as a Spring TaskExecutor.

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This class implements Spring's TaskExecutor interface as well as the Executor interface, with the former being the primary interface, the other just serving as secondary convenience. For this reason, the exception handling follows the TaskExecutor contract rather than the Executor contract, in particular regarding the TaskRejectedException.

Notice that ThreadPoolTaskExecutor implements many Spring interfaces like Aware, BeanNameAware, DisposableBean, InitializingBean which makes it easier to work with such pool as a Spring bean.

Also have a look at Karol Dowbecki's answer which correctly points out differences on those pools parameters.

In your example Spring's ThreadPoolTaskExecutor will create a ThreadPoolExecutor with corePoolSize of 50, maxPoolSize of Integer.MAX_VALUE and keepAlive of 60 seconds.

Meanwhile Executors.newFixedThreadPool(50) will set both corePoolSize and maxPoolSize to 50 and keepAlive of 0 seconds (see Java source code).