从源码角度分析Spring batch里ItemReader的使用

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：



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：



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



那reader可如下配置:

<bean id="playerFileItemReader" class="org.springframework.batch.item.file.FlatFileItemReader">
<property name="resource" value="classpath:data/footballjob/input/${player.file.name}" />
<property name="lineMapper">
<bean class="org.springframework.batch.item.file.mapping.DefaultLineMapper">
<property name="lineTokenizer">
<bean class="org.springframework.batch.item.file.transform.DelimitedLineTokenizer">
<property name="names" value="ID,lastName,firstName,position,birthYear,debutYear" />
</bean>
</property>
<property name="fieldSetMapper">
<bean class="org.springframework.batch.sample.domain.football.internal.PlayerFieldSetMapper" />
</property>
</bean>
</property>
</bean>

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



及其父类AbstractCursorItemReader：



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：



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



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：

<bean id="itemReader"
class="org.springframework.batch.item.database.IbatisPagingItemReader">
<property name="queryId" value="getAllCustomerCredits" />
<property name="sqlMapClient" ref="sqlMapClient" />
<property name="dataSource" ref="dataSource"/>
</bean>
<bean id="sqlMapClient" class="com.ibatis.sqlmap.client.SqlMapClientBuilder" factory-method="buildSqlMapClient">
<constructor-arg value="ibatis-config.xml"/>
</bean>

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
c90a
.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。