11. Common Batch Patterns
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## 11. Common Batch Patterns

Some batch jobs can be assembled purely from off-the-shelf components in Spring Batch. For instance the `ItemReader` and `ItemWriter` implementations can be configured to cover a wide range of scenarios. However, for the majority of cases, custom code will have to be written. The main API entry points for application developers are the `Tasklet`, `ItemReader`, `ItemWriter` and the various listener interfaces. Most simple batch jobs will be able to use off-the-shelf input from a Spring Batch `ItemReader`, but it is often the case that there are custom concerns in the processing and writing, which require developers to implement an `ItemWriter` or `ItemProcessor`.

Here, we provide a few examples of common patterns in custom business logic. These examples primarily feature the listener interfaces. It should be noted that an `ItemReader` or `ItemWriter` can implement a listener interface as well, if appropriate.

## 11.1 Logging Item Processing and Failures

A common use case is the need for special handling of errors in a step, item by item, perhaps logging to a special channel, or inserting a record into a database. A chunk-oriented `Step` (created from the step factory beans) allows users to implement this use case with a simple `ItemReadListener`, for errors on read, and an `ItemWriteListener`, for errors on write. The below code snippets illustrate a listener that logs both read and write failures:

    public class ItemFailureLoggerListener extends ItemListenerSupport {

        private static Log logger = LogFactory.getLog("item.error");

        public void onReadError(Exception ex) {
            logger.error("Encountered error on read", e);
        }

        public void onWriteError(Exception ex, Object item) {
            logger.error("Encountered error on write", e);
        }

    }

Having implemented this listener it must be registered with the step:

    <step id="simpleStep">
        ...
        <listeners>
            <listener>
                <bean class="org.example...ItemFailureLoggerListener"/>
            </listener>
        </listeners>
    </step>

Remember that if your listener does anything in an `onError()` method, it will be inside a transaction that is going to be rolled back. If you need to use a transactional resource such as a database inside an `onError()` method, consider adding a declarative transaction to that method (see Spring Core Reference Guide for details), and giving its propagation attribute the value REQUIRES_NEW.

## 11.2 Stopping a Job Manually for Business Reasons

Spring Batch provides a `stop`() method through the `JobLauncher` interface, but this is really for use by the operator rather than the application programmer. Sometimes it is more convenient or makes more sense to stop a job execution from within the business logic.

The simplest thing to do is to throw a `RuntimeException` (one that isn't retried indefinitely or skipped). For example, a custom exception type could be used, as in the example below:

    public class PoisonPillItemWriter implements ItemWriter<T> {

        public void write(T item) throws Exception {
            if (isPoisonPill(item)) {
                throw new PoisonPillException("Posion pill detected: " + item);
           }
        }

    }

Another simple way to stop a step from executing is to simply return `null` from the `ItemReader`:

    public class EarlyCompletionItemReader implements ItemReader<T> {

        private ItemReader<T> delegate;

        public void setDelegate(ItemReader<T> delegate) { ... }

        public T read() throws Exception {
            T item = delegate.read();
            if (isEndItem(item)) {
                return null; // end the step here
            }
            return item;
        }

    }

The previous example actually relies on the fact that there is a default implementation of the `CompletionPolicy` strategy which signals a complete batch when the item to be processed is null. A more sophisticated completion policy could be implemented and injected into the `Step` through the `SimpleStepFactoryBean`:

    <step id="simpleStep">
        <tasklet>
            <chunk reader="reader" writer="writer" commit-interval="10"
                   chunk-completion-policy="completionPolicy"/>
        </tasklet>
    </step>

    <bean id="completionPolicy" class="org.example...SpecialCompletionPolicy"/>

An alternative is to set a flag in the `StepExecution`, which is checked by the `Step` implementations in the framework in between item processing. To implement this alternative, we need access to the current `StepExecution`, and this can be achieved by implementing a `StepListener` and registering it with the `Step`. Here is an example of a listener that sets the flag:

    public class CustomItemWriter extends ItemListenerSupport implements StepListener {

        private StepExecution stepExecution;

        public void beforeStep(StepExecution stepExecution) {
            this.stepExecution = stepExecution;
        }

        public void afterRead(Object item) {
            if (isPoisonPill(item)) {
                stepExecution.setTerminateOnly(true);
           }
        }

    }

The default behavior here when the flag is set is for the step to throw a `JobInterruptedException`. This can be controlled through the `StepInterruptionPolicy`, but the only choice is to throw or not throw an exception, so this is always an abnormal ending to a job.

## 11.3 Adding a Footer Record

Often when writing to flat files, a "footer" record must be appended to the end of the file, after all processing has be completed. This can also be achieved using the `FlatFileFooterCallback` interface provided by Spring Batch. The `FlatFileFooterCallback` (and its counterpart, the `FlatFileHeaderCallback`) are optional properties of the `FlatFileItemWriter`:

    <bean id="itemWriter" class="org.spr...FlatFileItemWriter">
        <property name="resource" ref="outputResource" />
        <property name="lineAggregator" ref="lineAggregator"/>
        <property name="headerCallback" ref="headerCallback" />
        <property name="footerCallback" ref="footerCallback" />
    </bean>

The footer callback interface is very simple. It has just one method that is called when the footer must be written:

    public interface FlatFileFooterCallback {

        void writeFooter(Writer writer) throws IOException;

    }

### 11.3.1 Writing a Summary Footer

A very common requirement involving footer records is to aggregate information during the output process and to append this information to the end of the file. This footer serves as a summarization of the file or provides a checksum.

For example, if a batch job is writing `Trade` records to a flat file, and there is a requirement that the total amount from all the `Trade`s is placed in a footer, then the following `ItemWriter` implementation can be used:

    public class TradeItemWriter implements ItemWriter<Trade>,
                                            FlatFileFooterCallback {

        private ItemWriter<Trade> delegate;

        private BigDecimal totalAmount = BigDecimal.ZERO;

        public void write(List<? extends Trade> items) {
            BigDecimal chunkTotal = BigDecimal.ZERO;
            for (Trade trade : items) {
                chunkTotal = chunkTotal.add(trade.getAmount());
            }

            delegate.write(items);

            // After successfully writing all items
            totalAmount = totalAmount.add(chunkTotal);
        }

        public void writeFooter(Writer writer) throws IOException {
            writer.write("Total Amount Processed: " + totalAmount);
        }

        public void setDelegate(ItemWriter delegate) {...}
    }

This `TradeItemWriter` stores a `totalAmount` value that is increased with the `amount` from each `Trade` item written. After the last `Trade` is processed, the framework will call `writeFooter`, which will put that `totalAmount` into the file. Note that the `write` method makes use of a temporary variable, `chunkTotalAmount`, that stores the total of the trades in the chunk. This is done to ensure that if a skip occurs in the `write` method, that the totalAmount will be left unchanged. It is only at the end of the `write` method, once we are guaranteed that no exceptions will be thrown, that we update the `totalAmount`.

In order for the `writeFooter` method to be called, the `TradeItemWriter` (which implements `FlatFileFooterCallback`) must be wired into the `FlatFileItemWriter` as the `footerCallback`:

    <bean id="tradeItemWriter" class="..TradeItemWriter">
        <property name="delegate" ref="flatFileItemWriter" />
    </bean>

    <bean id="flatFileItemWriter" class="org.spr...FlatFileItemWriter">
       <property name="resource" ref="outputResource" />
       <property name="lineAggregator" ref="lineAggregator"/>
       <property name="footerCallback" ref="tradeItemWriter" />
    </bean>

The way that the `TradeItemWriter` has been so far will only function correctly if the `Step` is not restartable. This is because the class is stateful (since it stores the `totalAmount`), but the `totalAmount` is not persisted to the database, and therefore, it cannot be retrieved in the event of a restart. In order to make this class restartable, the `ItemStream` interface should be implemented along with the methods `open` and `update`:

    public void open(ExecutionContext executionContext) {
        if (executionContext.containsKey("total.amount") {
            totalAmount = (BigDecimal) executionContext.get("total.amount");
        }
    }

    public void update(ExecutionContext executionContext) {
        executionContext.put("total.amount", totalAmount);
    }

The `update` method will store the most current version of `totalAmount` to the `ExecutionContext` just before that object is persisted to the database. The `open` method will retrieve any existing `totalAmount` from the `ExecutionContext` and use it as the starting point for processing, allowing the `TradeItemWriter` to pick up on restart where it left off the previous time the `Step` was executed.

## 11.4 Driving Query Based ItemReaders

In the chapter on readers and writers, database input using paging was discussed. Many database vendors, such as DB2, have extremely pessimistic locking strategies that can cause issues if the table being read also needs to be used by other portions of the online application. Furthermore, opening cursors over extremely large datasets can cause issues on certain vendors. Therefore, many projects prefer to use a 'Driving Query' approach to reading in data. This approach works by iterating over keys, rather than the entire object that needs to be returned, as the following example illustrates:

As you can see, this example uses the same 'FOO' table as was used in the cursor based example. However, rather than selecting the entire row, only the ID's were selected in the SQL statement. So, rather than a FOO object being returned from `read`, an Integer will be returned. This number can then be used to query for the 'details', which is a complete Foo object:

An ItemProcessor should be used to transform the key obtained from the driving query into a full 'Foo' object. An existing DAO can be used to query for the full object based on the key.

## 11.5 Multi-Line Records

While it is usually the case with flat files that one each record is confined to a single line, it is common that a file might have records spanning multiple lines with multiple formats. The following excerpt from a file illustrates this:

    HEA;0013100345;2007-02-15
    NCU;Smith;Peter;;T;20014539;F
    BAD;;Oak Street 31/A;;Small Town;00235;IL;US
    FOT;2;2;267.34

Everything between the line starting with 'HEA' and the line starting with 'FOT' is considered one record. There are a few considerations that must be made in order to handle this situation correctly:

- Instead of reading one record at a time, the `ItemReader` must read every line of the multi-line record as a group, so that it can be passed to the `ItemWriter` intact.

- Each line type may need to be tokenized differently.

Because a single record spans multiple lines, and we may not know how many lines there are, the `ItemReader` must be careful to always read an entire record. In order to do this, a custom `ItemReader` should be implemented as a wrapper for the `FlatFileItemReader`.

    <bean id="itemReader" class="org.spr...MultiLineTradeItemReader">
        <property name="delegate">
            <bean class="org.springframework.batch.item.file.FlatFileItemReader">
                <property name="resource" value="data/iosample/input/multiLine.txt" />
                <property name="lineMapper">
                    <bean class="org.spr...DefaultLineMapper">
                        <property name="lineTokenizer" ref="orderFileTokenizer"/>
                        <property name="fieldSetMapper">
                            <bean class="org.spr...PassThroughFieldSetMapper" />
                        </property>
                    </bean>
                </property>
            </bean>
        </property>
    </bean>

To ensure that each line is tokenized properly, which is especially important for fixed length input, the `PatternMatchingCompositeLineTokenizer` can be used on the delegate `FlatFileItemReader`. See the section called “Multiple Record Types within a Single File” for more details. The delegate reader will then use a `PassThroughFieldSetMapper` to deliver a `FieldSet` for each line back to the wrapping `ItemReader`.

    <bean id="orderFileTokenizer" class="org.spr...PatternMatchingCompositeLineTokenizer">
        <property name="tokenizers">
            <map>
                <entry key="HEA*" value-ref="headerRecordTokenizer" />
                <entry key="FOT*" value-ref="footerRecordTokenizer" />
                <entry key="NCU*" value-ref="customerLineTokenizer" />
                <entry key="BAD*" value-ref="billingAddressLineTokenizer" />
            </map>
        </property>
    </bean>

This wrapper will have to be able recognize the end of a record so that it can continually call `read()` on its delegate until the end is reached. For each line that is read, the wrapper should build up the item to be returned. Once the footer is reached, the item can be returned for delivery to the `ItemProcessor` and `ItemWriter`.

    private FlatFileItemReader<FieldSet> delegate;

    public Trade read() throws Exception {
        Trade t = null;

        for (FieldSet line = null; (line = this.delegate.read()) != null;) {
            String prefix = line.readString(0);
            if (prefix.equals("HEA")) {
                t = new Trade(); // Record must start with header
            }
            else if (prefix.equals("NCU")) {
                Assert.notNull(t, "No header was found.");
                t.setLast(line.readString(1));
                t.setFirst(line.readString(2));
                ...
            }
            else if (prefix.equals("BAD")) {
                Assert.notNull(t, "No header was found.");
                t.setCity(line.readString(4));
                t.setState(line.readString(6));
              ...
            }
            else if (prefix.equals("FOT")) {
                return t; // Record must end with footer
            }
        }
        Assert.isNull(t, "No 'END' was found.");
        return null;
    }

## 11.6 Executing System Commands

Many batch jobs may require that an external command be called from within the batch job. Such a process could be kicked off separately by the scheduler, but the advantage of common meta-data about the run would be lost. Furthermore, a multi-step job would also need to be split up into multiple jobs as well.

Because the need is so common, Spring Batch provides a `Tasklet` implementation for calling system commands:

    <bean class="org.springframework.batch.core.step.tasklet.SystemCommandTasklet">
        <property name="command" value="echo hello" />
        <!-- 5 second timeout for the command to complete -->
        <property name="timeout" value="5000" />
    </bean>

## 11.7 Handling Step Completion When No Input is Found

In many batch scenarios, finding no rows in a database or file to process is not exceptional. The `Step` is simply considered to have found no work and completes with 0 items read. All of the `ItemReader` implementations provided out of the box in Spring Batch default to this approach. This can lead to some confusion if nothing is written out even when input is present. (which usually happens if a file was misnamed, etc) For this reason, the meta data itself should be inspected to determine how much work the framework found to be processed. However, what if finding no input is considered exceptional? In this case, programmatically checking the meta data for no items processed and causing failure is the best solution. Because this is a common use case, a listener is provided with just this functionality:

    public class NoWorkFoundStepExecutionListener extends StepExecutionListenerSupport {

        public ExitStatus afterStep(StepExecution stepExecution) {
            if (stepExecution.getReadCount() == 0) {
                return ExitStatus.FAILED;
            }
            return null;
        }

    }

The above `StepExecutionListener` inspects the readCount property of the `StepExecution` during the 'afterStep' phase to determine if no items were read. If that is the case, an exit code of FAILED is returned, indicating that the `Step` should fail. Otherwise, null is returned, which will not affect the status of the `Step`.

## 11.8 Passing Data to Future Steps

It is often useful to pass information from one step to another. This can be done using the `ExecutionContext`. The catch is that there are two `ExecutionContext`s: one at the `Step` level and one at the `Job` level. The `Step` `ExecutionContext` lives only as long as the step while the `Job` `ExecutionContext` lives through the whole `Job`. On the other hand, the `Step` `ExecutionContext` is updated every time the `Step` commits a chunk while the `Job` `ExecutionContext` is updated only at the end of each `Step`.

The consequence of this separation is that all data must be placed in the `Step` `ExecutionContext` while the `Step` is executing. This will ensure that the data will be stored properly while the `Step` is on-going. If data is stored to the `Job` `ExecutionContext`, then it will not be persisted during `Step` execution and if the `Step` fails, that data will be lost.

    public class SavingItemWriter implements ItemWriter<Object> {
        private StepExecution stepExecution;

        public void write(List<? extends Object> items) throws Exception {
            // ...

            ExecutionContext stepContext = this.stepExecution.getExecutionContext();
            stepContext.put("someKey", someObject);
        }

        @BeforeStep
        public void saveStepExecution(StepExecution stepExecution) {
            this.stepExecution = stepExecution;
        }
    }

To make the data available to future `Step`s, it will have to be "promoted" to the `Job` `ExecutionContext` after the step has finished. Spring Batch provides the `ExecutionContextPromotionListener` for this purpose. The listener must be configured with the keys related to the data in the `ExecutionContext` that must be promoted. It can also, optionally, be configured with a list of exit code patterns for which the promotion should occur ("COMPLETED" is the default). As with all listeners, it must be registered on the `Step`.

    <job id="job1">
        <step id="step1">
            <tasklet>
                <chunk reader="reader" writer="savingWriter" commit-interval="10"/>
            </tasklet>
            <listeners>
                <listener ref="promotionListener"/>
            </listeners>
        </step>

        <step id="step2">
           ...
        </step>
    </job>

    <beans:bean id="promotionListener" class="org.spr....ExecutionContextPromotionListener">
        <beans:property name="keys" value="someKey"/>
    </beans:bean>

Finally, the saved values must be retrieved from the `Job` `ExeuctionContext`:

    public class RetrievingItemWriter implements ItemWriter<Object> {
        private Object someObject;

        public void write(List<? extends Object> items) throws Exception {
            // ...
        }

        @BeforeStep
        public void retrieveInterstepData(StepExecution stepExecution) {
            JobExecution jobExecution = stepExecution.getJobExecution();
            ExecutionContext jobContext = jobExecution.getExecutionContext();
            this.someObject = jobContext.get("someKey");
        }
    }

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