Spring Batch：使用ClassifierCompositeItemWriter避免空文件写入表头

项目采用Spring XML配置，使用ClassifierCompositeItemWriter根据某列数据分类，将数据写入四个不同文件（每个文件数据唯一）。当前遇到的问题是：当某个文件没有符合分类器的数据时，仍会写入表头，希望这类空文件保持纯空状态，不生成表头。

现有配置及代码如下：

XML配置 #

<bean id="someCvsWriter" class="org.springframework.batch.item.file.FlatFileItemWriter">
    <property name="resource" value="file:csv/someReport.csv"/>
    <property name="shouldDeleteIfExists" value="true"/>
    <property name="encoding" value="UTF-8"/>
    <property name="lineAggregator" ref="someCustomLineAggregator"/>
    <property name="headerCallback" ref="someReportHeaderCallback" />
</bean>

HeaderCallback实现类 #

public class SomeReportFlatFileHeaderCallback implements FlatFileHeaderCallback {

    @Override
    public void writeHeader(Writer writer) throws IOException {
        writer.write("COL1, COL2, COL3");
    }
}

方法1：扩展FlatFileItemWriter重写表头逻辑 #

直接扩展FlatFileItemWriter，内部跟踪是否有数据写入，仅在有数据时生成表头，无需额外依赖：

public class ConditionalHeaderFlatFileItemWriter<T> extends FlatFileItemWriter<T> {
    private boolean hasWrittenItems = false;

    @Override
    public void write(List<? extends T> items) throws Exception {
        if (!items.isEmpty()) {
            hasWrittenItems = true;
            super.write(items);
        }
    }

    @Override
    protected void writeHeader(Writer writer) throws IOException {
        // 仅当有数据写入时才输出表头
        if (hasWrittenItems) {
            super.writeHeader(writer);
        }
    }

    @Override
    public void open(ExecutionContext executionContext) {
        super.open(executionContext);
        // 每次作业启动时重置标记
        hasWrittenItems = false;
    }
}

修改XML配置，替换原FlatFileItemWriter为自定义版本：

<bean id="someCvsWriter" class="com.yourpackage.ConditionalHeaderFlatFileItemWriter">
    <property name="resource" value="file:csv/someReport.csv"/>
    <property name="shouldDeleteIfExists" value="true"/>
    <property name="encoding" value="UTF-8"/>
    <property name="lineAggregator" ref="someCustomLineAggregator"/>
    <property name="headerCallback" ref="someReportHeaderCallback" />
</bean>

方法2：自定义条件HeaderCallback（适合需复用原Header逻辑场景） #

如果需要保留原HeaderCallback的逻辑，可通过计数Bean判断是否输出表头：

1. 定义数据计数跟踪类

public class ItemCountTracker {
    private int count = 0;

    public void increment() {
        count++;
    }

    public boolean hasItems() {
        return count > 0;
    }

    public void reset() {
        count = 0;
    }
}

2. 实现条件HeaderCallback

public class ConditionalHeaderCallback implements FlatFileHeaderCallback {
    private FlatFileHeaderCallback delegate;
    private ItemCountTracker itemCountTracker;

    public ConditionalHeaderCallback(FlatFileHeaderCallback delegate, ItemCountTracker itemCountTracker) {
        this.delegate = delegate;
        this.itemCountTracker = itemCountTracker;
    }

    @Override
    public void writeHeader(Writer writer) throws IOException {
        if (itemCountTracker.hasItems()) {
            delegate.writeHeader(writer);
        }
    }
}

3. 包装原Writer实现计数

public class CountingItemWriter<T> implements ItemWriter<T> {
    private ItemWriter<T> delegate;
    private ItemCountTracker itemCountTracker;

    public CountingItemWriter(ItemWriter<T> delegate, ItemCountTracker itemCountTracker) {
        this.delegate = delegate;
        this.itemCountTracker = itemCountTracker;
    }

    @Override
    public void write(List<? extends T> items) throws Exception {
        if (!items.isEmpty()) {
            itemCountTracker.increment();
            delegate.write(items);
        }
    }
}

4. 修改XML配置

<!-- 每个Writer对应独立的计数Bean，step作用域保证每次作业重置 -->
<bean id="someItemCountTracker" class="com.yourpackage.ItemCountTracker" scope="step"/>

<!-- 条件HeaderCallback依赖原Header和计数Bean -->
<bean id="conditionalSomeHeaderCallback" class="com.yourpackage.ConditionalHeaderCallback">
    <constructor-arg ref="someReportHeaderCallback"/>
    <constructor-arg ref="someItemCountTracker"/>
</bean>

<!-- 包装原Writer为计数Writer -->
<bean id="countingSomeCvsWriter" class="com.yourpackage.CountingItemWriter">
    <constructor-arg ref="someCvsWriter"/>
    <constructor-arg ref="someItemCountTracker"/>
</bean>

<!-- 更新原Writer的headerCallback为条件版本 -->
<bean id="someCvsWriter" class="org.springframework.batch.item.file.FlatFileItemWriter">
    <property name="resource" value="file:csv/someReport.csv"/>
    <property name="shouldDeleteIfExists" value="true"/>
    <property name="encoding" value="UTF-8"/>
    <property name="lineAggregator" ref="someCustomLineAggregator"/>
    <property name="headerCallback" ref="conditionalSomeHeaderCallback" />
</bean>

内容的提问来源于stack exchange，提问作者PAA