精通Spring Batch：Java批量处理指南

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"Pro Spring Batch Java Spring Batch pdf" 《Pro Spring Batch》这本书是关于Java Spring框架中的批处理技术的专业指南。Spring Batch是Spring生态系统的组件，专门用于处理批量数据处理任务。本书旨在深入介绍Spring Batch的基础知识、核心概念以及高级用法，帮助开发者高效地构建可扩展和可维护的批处理应用。书中内容涵盖了以下关键知识点： 1. **批处理和Spring**：第一章介绍了批处理的基本概念，并阐述了Spring框架如何为批处理提供支持。它解释了Spring Batch在Java世界中的重要性，以及如何将批处理任务集成到Spring应用中。 2. **Spring Batch 101**：第二章是Spring Batch的基础教程，包括其核心组件、配置和基本工作流程。读者将了解到如何创建和配置Job、Step等关键元素。 3. **样本Job**：第三章通过一个实际的Job示例，展示了Spring Batch如何处理批处理任务的生命周期，包括启动、执行和终止。 4. **理解Jobs和Steps**：第四章深入探讨Job和Step的概念，解释它们之间的关系以及如何定义和管理它们的执行流程。 5. **JobRepository和元数据**：第五章讲解了JobRepository的作用，它是存储批处理运行状态和元数据的关键。这部分内容包括如何持久化作业状态和执行历史。 6. **运行Job**：第六章指导读者如何在不同的环境中启动和监控Job，包括命令行、Spring Boot应用和Web应用中的集成。 7. **Readers**：第七章专注于数据读取器（Readers），这是批处理流程的第一步，讨论了各种类型的ItemReader实现，如FlatFileItemReader和JDBCItemReader。 8. **ItemProcessors**：第八章介绍ItemProcessor，这是处理数据的核心组件，讲解如何自定义处理器以实现业务逻辑。 9. **ItemWriters**：第九章关注数据写入器（ItemWriters），讨论如何将处理后的数据写入目标系统，如数据库、文件系统或其他服务。 10. **样本应用程序**：第十章提供了一个完整的批处理应用实例，展示了如何将前面章节学到的知识整合到实际项目中。 11. **扩展与调优**：第十一章讨论了Spring Batch的可扩展性和性能调优，包括并行处理、分布式执行和错误处理策略。 12. **测试批处理过程**：第十二章介绍了如何对批处理过程进行单元测试和集成测试，确保代码质量和批处理任务的正确性。 13. **索引**：最后，书后附有完整的索引，方便读者查找特定主题。这本书对于任何想要深入了解和使用Spring Batch来处理大量数据的Java开发者来说，都是宝贵的资源。它不仅提供了理论知识，还包含了大量的实践案例和技巧，有助于读者在实际工作中有效应用Spring Batch。

CHAPTER 2  SPRING BATCH 101

• org.springframework.batch.core.Step

Like the job, represents the step as

configured in the XML as well as

provides the ability to execute a step.

• org.springframework.batch.item.ItemReader<T>

A strategy interface that provides the

ability to input items.

• org.springframework.batch.item.ItemProcessor<T>

A facility to apply business logic to an

individual item as provided.

• org.springframework.batch.item.ItemWriter<T>

A strategy interface that provides the

ability to output a list of items.

One of the advantages of the way Spring has structured a job is that it decouples each step into its

own independent processor. Each step is responsible for obtaining its own data, applying the required

business logic to it, and then writing the data to the appropriate location. This decoupling provides a

number of features:

• Flexibility: The ability to alter the order of processing with nothing more than an

XML change is something many frameworks talk about yet very few deliver. Spring

Batch is one that does deliver. Thinking about the earlier bank account example.,

If you wanted to apply the debits before the credits, the only change required

would be to reorder the steps in the job XML (Chapter 4 shows an example). You

can also skip a step, execute a step conditionally based on the results of a previous

step, or even run multiple steps in parallel by doing nothing more than tweaking

the XML.

• Maintainability: With the code for each step decoupled from the steps before and

after it, steps are easy to unit-test, debug, and update with virtually no impact on

other steps. Decoupled steps also make it possible to reuse steps in multiple jobs.

As you’ll see in upcoming chapters, steps are nothing more than Spring beans and

can be reused just like any other bean in Spring.

• Scalability: Decoupling steps in a job provides a number of options to scale your

jobs. You can execute steps in parallel. You can divide the work within a step

across threads and execute the code of a single step in parallel (you see a bit more

about this later in the chapter). Any of these abilities lets you meet the scalability

needs of your business with minimum direct impact on your code.

• Reliability: By decoupling each step and each piece within a step, you can

structure jobs such that they can be restarted at a given point in the process. If a

job fails after processing 50,000 records out of 10 million in step 3 out of 7, you can

restart it right where it left off.

Job Execution

Let’s look at what happens with the components and their relationships when a job is run. Notice in

Figure 2-2 that the piece most of the components share is JobRepository. This is a datastore (in memory

or a database) that is used to persist information about the job and step executions. A JobExecution or

CHAPTER 2  SPRING BATCH 101

StepExecution is information about a single run of the job or step. You see more detail about what is in

the executions and the repository later in this chapter and in Chapter 5.

Job

JobRepository

Step

ItemReader

JobLauncher

ItemProcessor

ItemWriter

Figure 2-2. The job components and their relationships

Running a job begins with the JobLauncher. The JobLauncher verifies whether the job has been run

before by checking the JobRepository, validates the parameters being passed into the job, and, finally,

executes the job.

The processing of a job and a step are very similar. A job goes through the list of steps it has been

configured to run, executing each one. As a chunk of items completes, Spring Batch updates the

JobExecution or StepExecution in the repository with the results of the execution. A step goes through a

list of items as read in by the ItemReader. As the step processes each chunk of items, the StepExecution

in the repository is updated with where it is in the step. Things like current commit count, start and end

times, and other information are stored in the repository. When a job or step is complete, the related

execution is updated in the repository with the final status.

One of the things that changed in Spring Batch from version 1 to 2 was the addition of chunked

processing. In version 1, records were read in, processed, and written out one at a time. The issue with

this is that it doesn’t take advantage of the ability to batch-write that Java’s file and database I/O

provides (buffered writing and batch updates). In version 2 and beyond of Spring Batch, the framework

has been updated. Reading and processing is still a singular operation; there is no reason to load a large

amount of data into memory if it can’t be processed. But now, the write only occurs once a commit

count interval occurs. This allows for more performant writing of records as well as a more capable

rollback mechanism.

Parallelization

A simple batch process’s architecture consists of a single-threaded process that executes a job’s steps in

order from start to finish. However, Spring Batch provides a number of parallelization options that you

should be aware of as you move forward. (Chapter 11 covers these options in detail.) There are four

different ways to parallelize your work: dividing work via multithreaded steps, parallel execution of full

steps, remote chunking, and partitioning.

CHAPTER 2  SPRING BATCH 101

Remote Chunking

The last two approaches to parallelization allow you to spread processing across multiple JVMs. In all

cases previously, the processing was performed in a single JVM, which can seriously hinder the

scalability options. When you can scale any part of your process horizontally across multiple JVMs, the

ability to keep up with large demands increases.

The first remote-processing option is remote chunking. In this approach, input is performed using a

standard ItemReader in a master node; the input is then sent via a form of durable communication (JMS

for example) to a remote slave ItemProcessor that is configured as a message driven POJO. When the

processing is complete, the slave sends the updated item back to the master for writing. Because this

approach reads the data at the master, processes it at the slave, and then sends it back, it’s important to

note that it can be very network intensive. This approach is good for scenarios where the cost of I/O is

small compared to the actual processing.

Partitioning

The final method for parallelization within Spring Batch is partitioning, shown in Figure 2-5. Again, you

use a master/slave configuration; but this time you don’t need a durable method of communication, and

the master serves only as a controller for a collection of slave steps. In this case, each of your slave steps

is self-contained and configured the same as if it was locally deployed. The only difference is that the

slave steps receive their work from the master node instead of the job itself. When all the slaves have

completed their work, the master step is considered complete. This configuration doesn’t require

durable communication with guaranteed delivery because the JobRepository guarantees that no work is

duplicated and all work is completed—unlike the remote-chunking approach, in which the

JobRepository has no knowledge of the state of the distributed work.

Step 1

Step 2

Slave

Step 2

Slave

Step 2

Slave

Step 2

Master

Step 1

Figure 2-5. Partitioning work

Batch Administration

Any enterprise system must be able to start and stop processes, monitor their current state, and even

view results. With web applications, this is easy: in the web application, you see the results of each action

you request, and tools like Google Analytics provide various metrics on how your application is being

used and is performing.

However, in the batch world, you may have a single Java process running on a server for eight hours

with no output other than log files and the database the process is working on. This is hardly a

manageable situation. For this reason, Spring has developed a web application called Spring Batch

Admin that lets you start and stop jobs and also provides details about each job execution.

CHAPTER 2  SPRING BATCH 101

Documentation

One of the strengths of Spring Batch is that real developers wrote it who have experience developing

batch processes in various enterprises. From this experience comes not only a comprehensive

framework but also a wealth of documentation to boot. The Spring Batch web site contains one of the

best collections of documentation for an open source project I’ve ever worked with. Along with the

formal documentation, the JavaDoc is also useful for API specifics. Finally, Spring Batch provides 19

different sample jobs for you to reference as you develop your own batch applications (see Table 2-2).

Table 2-2. Sample Batch Jobs

Batch Job Description

adhocLoopJob An infinite loop used to demonstrate the exposing of elements via

JMX and the running of the job in a background thread (instead of

the main JobLauncher thread).

beanWrapperMapperSampleJob A job with two steps that is used to demonstrate the mapping of

file fields to domain objects as well as validation of file-based

input.

compositeItemWriterSampleJob A step can have only one reader and writer. The CompositeWriter

is the way around this. This sample job demonstrates how.

customerFilterJob Uses an ItemProcessor to filter out customers that aren’t valid.

This job also updates the filter count field of the step execution.

delegatingJob Using the ItemReaderAdapter, delegates the reading of input to a

configured method of a POJO.

footballJob A football statistics job. After loading two input files, one with

player data and one with game data, the job generates a selection

of summary statistics for the players and games and writes them

to the log file.

groovyJob Uses Groovy (a dynamic JVM language) to script the unzipping

and zipping of a file.

headerFooterSample Using callbacks, adds the ability to render a header and footer on

the output.

hibernateJob Spring Batch readers and writers don’t use Hibernate by default.

This job shows how to integrate Hibernate into your job.

infiniteLoopJob Just a job with an infinite loop, used to demonstrate stop and

restart scenarios

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