Spring Batch_Multi-threaded Step_使用多线程的Step

Spring Batch_Multi-threaded Step_使用多线程的Step

spring官方文档：http://docs.spring.io/spring-batch/trunk/reference/html/scalability.html

The simplest way to start parallel processing is to add a TaskExecutor to your Step configuration, e.g. as an attribute of the tasklet:

<step id="loading">
    <tasklet task-executor="taskExecutor">...</tasklet>
</step>

In this example the taskExecutor is a reference to another bean definition, implementing the TaskExecutor interface. TaskExecutor is a standard Spring interface, so consult（查阅，参照） the Spring User Guide for details of available implementations. The simplest multi-threaded TaskExecutor is a SimpleAsyncTaskExecutor.

The result of the above configuration will be that the Step executes by reading, processing and writing each chunk of items (each commit interval) in a separate thread of execution. Note that this means there is no fixed order for the items to be processed, and a chunk might contain items that are non-consecutive（不连续的） compared to the single-threaded case. In addition to any limits placed by the task executor (e.g. if it is backed by a thread pool), there is a throttle limit in the tasklet configuration which defaults to 4. You may need to increase this to ensure that a thread pool is fully utilised, e.g.

<step id="loading"> <tasklet
    task-executor="taskExecutor"
    throttle-limit="20">...</tasklet>
</step>

Note also that there may be limits placed on concurrency by any pooled resources used in your step, such as a DataSource. Be sure to make the pool in those resources at least as large as the desired number of concurrent threads in the step.

There are some practical limitations of using multi-threaded Steps for some common Batch use cases. Many participants in a Step (e.g. readers and writers) are stateful（有状态的）, and if the state is not segregated（分离，隔开的） by thread, then those components are not usable in a multi-threaded Step. In particular most of the off-the-shelf（现成的） readers and writers from Spring Batch are not designed for multi-threaded use. It is, however, possible to work with stateless or thread safe readers and writers, and there is a sample (parallelJob) in the Spring Batch Samples that show the use of a process indicator (see Section 6.12, “Preventing State Persistence”) to keep track of items that have been processed in a database input table.

Spring Batch provides some implementations of ItemWriter and ItemReader. Usually they say in the Javadocs if they are thread safe or not, or what you have to do to avoid problems in a concurrent environment. If there is no information in Javadocs, you can check the implementation to see if there is any state. If a reader is not thread safe, it may still be efficient（有效率的） to use it in your own synchronizing delegator. You can synchronize the call to read() and as long as the processing and writing is the most expensive part of the chunk your step may still complete much faster than in a single threaded configuration.

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