从源码角度分析Spring batch里ItemReader的使用

亦凉 2022-07-03 16:57 474阅读 0赞

想要成为技术大牛，必需要看源码，从源码分析。下面分析几种springBatch里的ItemReader子类。

ItemReader<T>(泛型)是所有Reader的父类。且只有一个方法Tread() throws Exception, UnexpectedInputException, ParseException,NonTransientResourceException;

所以子类的参数或通过spring注入或创建默认。

所有子类如下图：

![Center][]

挑几个常用的做介绍。

文件读取类

FlatFileItemReader：文件读取类，属性及方法如下：

![Center 1][]

原来该类通过BufferedReader一行一行读取文件数据，同时以\#开头的视做注释，跳过读取。真正做事的是doRead方法。而doRead():

protected T doRead() throws Exception {
    		if (noInput) {
    			return null;
    		}
    
    		String line = readLine();
    
    		if (line == null) {
    			return null;
    		}
    		else {
    			try {
    				return lineMapper.mapLine(line, lineCount);
    			}
    			catch (Exception ex) {
    				throw new FlatFileParseException("Parsing error at line: " + lineCount + " in resource=["
    						+ resource.getDescription() + "], input=[" + line + "]", ex, line, lineCount);
    			}
    		}
    	}

在读取之前会有打开的操作doOpen():

protected void doOpen() throws Exception {
    		Assert.notNull(resource, "Input resource must be set");
    		Assert.notNull(recordSeparatorPolicy, "RecordSeparatorPolicy must be set");
    
    		noInput = true;
    		if (!resource.exists()) {
    			if (strict) {
    				throw new IllegalStateException("Input resource must exist (reader is in 'strict' mode): " + resource);
    			}
    			logger.warn("Input resource does not exist " + resource.getDescription());
    			return;
    		}
    
    		if (!resource.isReadable()) {
    			if (strict) {
    				throw new IllegalStateException("Input resource must be readable (reader is in 'strict' mode): "
    						+ resource);
    			}
    			logger.warn("Input resource is not readable " + resource.getDescription());
    			return;
    		}
    
    		reader = bufferedReaderFactory.create(resource, encoding);
    		for (int i = 0; i < linesToSkip; i++) {
    			String line = readLine();
    			if (skippedLinesCallback != null) {
    				skippedLinesCallback.handleLine(line);
    			}
    		}
    		noInput = false;
    	}

见红色字体，必需保证资源存在可读，同时初始化一个BufferedReader实例。

配置例子：

假设数据是  
![Center 2][]

那reader可如下配置:

这样数据和红色字段指定的names字段就会自动映射。

**故，想要使用FlatFileItemReader必需配resource和lineMapper两个属性，其它可选配。指定names字段，这样，字段映射成一个对象传给ItemWriter<T>**

数据库操作：

由于持久层有hiberate,ibatis,mybatis,所以不同的框架读取不一样:

游标读取方式：JdbcCursorItemReader。通过查看JdbcCursorItemReader

![Center 3][]  
及其父类AbstractCursorItemReader：

![Center 4][]

的代码，我们发现必要参数是sql, dataSource及rowMapper必需注入。

AbstractCursorItemReader.doRead():

protected T doRead() throws Exception {
    		if (rs == null) {
    			throw new ReaderNotOpenException("Reader must be open before it can be read.");
    		}
    	
    		try {
    			if (!rs.next()) {
    				return null;
    			}
    			int currentRow = getCurrentItemCount();
    			T item = readCursor(rs, currentRow);
    			verifyCursorPosition(currentRow);
    			return item;
    		}
    		catch (SQLException se) {
    			throw getExceptionTranslator().translate("Attempt to process next row failed", getSql(), se);
    		}
    	}

不难发现rs是执行sql语句返回的结果集，该方法返回当前游标的的item,注意类型是T即泛型。所以，dataSource指定数据源，sql即指定查询语句，rowMapper对当前游标位置的数据进行包装，得到的结果即是指定的结构对象，我一直强调泛型，因为我们用返回的结果即是泛型指定的类型。

配置例子：

<bean id="customerSqlItemReader" class="org.springframework.batch.item.database.JdbcCursorItemReader">
    		<property name="dataSource" ref="dataSource" />
    		<property name="sql" value="SELECT id, name, credit FROM CUSTOMER " />
    		<property name="rowMapper">
    			<bean class="org.springframework.batch.sample.domain.trade.internal.CustomerCreditRowMapper" />
    		</property>
    	</bean>
    CustomerCreditRowMapper.java:
    
    public class CustomerCreditRowMapper implements RowMapper<CustomerCredit> {
    	
    	public static final String ID_COLUMN = "id";
    	public static final String NAME_COLUMN = "name";
    	public static final String CREDIT_COLUMN = "credit";
    
    	@Override
    	public CustomerCredit mapRow(ResultSet rs, int rowNum) throws SQLException {
            CustomerCredit customerCredit = new CustomerCredit();
    
            customerCredit.setId(rs.getInt(ID_COLUMN));
            customerCredit.setName(rs.getString(NAME_COLUMN));
            customerCredit.setCredit(rs.getBigDecimal(CREDIT_COLUMN));
    
            return customerCredit;
    	}

Ibatis读取方式：

先看IbatisPagingItemReader的父类AbstractPagingItemReader类结构图：

![Center 5][]

分别可设置页号page,页尺寸pageSize默认10等，重点看看doRead()方法：

@Override
    	protected T doRead() throws Exception {
    
    		synchronized (lock) {
    
    			if (results == null || current >= pageSize) {
    
    				if (logger.isDebugEnabled()) {
    					logger.debug("Reading page " + getPage());
    				}
    
    				doReadPage();
    				page++;
    				if (current >= pageSize) {
    					current = 0;
    				}
    
    			}
    
    			int next = current++;
    			//返回下一个item
    			if (next < results.size()) {
    				return results.get(next);
    			}
    			else {
    				return null;
    			}
    
    		}
    
    	}

具体读操作由子类doReadPage方法完成，故现在看看IbatisPagingItemReader

![Center 6][]

配queryId(映射文件xml里的方法名称)，sqlMapCLient实例即是ibatis的配置文件，还要配dataSource.看看它是怎么完成查询的

@Override
    	@SuppressWarnings("unchecked")
    	protected void doReadPage() {
    		Map<String, Object> parameters = new HashMap<String, Object>();
    		if (parameterValues != null) {
    			parameters.putAll(parameterValues);
    		}
    		parameters.put("_page", getPage());
    		parameters.put("_pagesize", getPageSize());
    		parameters.put("_skiprows", getPage() * getPageSize());
    		if (results == null) {
    			results = new CopyOnWriteArrayList<T>();
    		}
    		else {
    			results.clear();
    		}
    		results.addAll(sqlMapClientTemplate.queryForList(queryId, parameters));//过程以目了然
    	}

所以IbatisPagingItemReader必需配的参数为queryId，sqlMapCLient， dataSource，其它选配。

配置例子：

ibatis-config.xml：

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE sqlMapConfig PUBLIC "-//ibatis.apache.org//DTD SQL Map Config 2.0//EN"
    	"http://ibatis.apache.org/dtd/sql-map-config-2.dtd">
    
    <sqlMapConfig>
      <sqlMap resource="ibatis-customer-credit.xml"/>
    </sqlMapConfig>

ibatis-customer-credit.xml：

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE sqlMap PUBLIC "-//ibatis.apache.org//DTD SQL Map 2.0//EN"
    	"http://ibatis.apache.org/dtd/sql-map-2.dtd">
    
    <sqlMap namespace="Customer">
      <resultMap id="result" class="org.springframework.batch.sample.domain.trade.CustomerCredit">
        <result property="name" column="NAME" />
        <result property="credit" column="CREDIT" />
      </resultMap>
    
      <select id="getAllCustomerCreditIds" resultClass="int">
        select ID from CUSTOMER
      </select>
      
      <select id="getAllCustomerCredits" resultMap="result">
     	select ID, NAME, CREDIT from CUSTOMER 
      </select>
      
      <select id="getCustomerCreditById" parameterClass="int" resultMap="result">
      	select NAME, CREDIT from CUSTOMER where ID = #value#
      </select>
      
      <update id="updateCredit" parameterClass="org.springframework.batch.sample.domain.trade.CustomerCredit" >
      	update CUSTOMER set CREDIT = #credit# where NAME = #name#
      </update>
    </sqlMap>

Ps:不需要对结果集包装，因为ibatis里将读取的数据映射。和spring-batch框架没关系

总结，以上只介绍了几咱读取的类，常用读取的要么文件要么数据库，会这两种类型即可，其实简单。

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