新老版本好像不太一样，此处为5.x版本
开始源码之旅~
始发站：
PageHelper.startPage(pageNum, pageSize);
进入startPage(pageNum, pageSize):
一路重载到：
public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {
Page<E> page = new Page(pageNum, pageSize, count);
page.setReasonable(reasonable);
page.setPageSizeZero(pageSizeZero);
Page<E> oldPage = getLocalPage();
if (oldPage != null && oldPage.isOrderByOnly()) {
page.setOrderBy(oldPage.getOrderBy());
}
setLocalPage(page);
return page;
}
这个方法将pageNum和pageSize用来构造了page，并调用setLocalPage()方法：
protected static final ThreadLocal<Page> LOCAL_PAGE = new ThreadLocal();
protected static void setLocalPage(Page page) {
LOCAL_PAGE.set(page);
}
将page放入了ThreadLocal中。
继续执行，进入MapperProxy类的invoke方法：
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
}
if (this.isDefaultMethod(method)) {
return this.invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable var5) {
throw ExceptionUtil.unwrapThrowable(var5);
}
MapperMethod mapperMethod = this.cachedMapperMethod(method);
return mapperMethod.execute(this.sqlSession, args);
}
继续执行，进入MapperMethod的execute方法：
这里根据不同的操作类型，进入不同的方法，在此进入查询多个的方法executeForMany
private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
Object param = this.method.convertArgsToSqlCommandParam(args);
List result;
if (this.method.hasRowBounds()) {
RowBounds rowBounds = this.method.extractRowBounds(args);
result = sqlSession.selectList(this.command.getName(), param, rowBounds);
} else {
result = sqlSession.selectList(this.command.getName(), param);
}
if (!this.method.getReturnType().isAssignableFrom(result.getClass())) {
return this.method.getReturnType().isArray() ? this.convertToArray(result) : this.convertToDeclaredCollection(sqlSession.getConfiguration(), result);
} else {
return result;
}
}
继续进入到sqlSession.selectList
再进到SqlSessionTemplate内部类SqlSessionInterceptor的invoke()方法：
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
SqlSession sqlSession = SqlSessionUtils.getSqlSession(SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);
Object unwrapped;
try {
Object result = method.invoke(sqlSession, args);
if (!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
sqlSession.commit(true);
}
unwrapped = result;
} catch (Throwable var11) {
unwrapped = ExceptionUtil.unwrapThrowable(var11);
if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
sqlSession = null;
Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException)unwrapped);
if (translated != null) {
unwrapped = translated;
}
}
throw (Throwable)unwrapped;
} finally {
if (sqlSession != null) {
SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
return unwrapped;
}
进入method.invoke()：
到达DefaultSqlSession的selectList()：
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
List var5;
try {
MappedStatement ms = this.configuration.getMappedStatement(statement);
var5 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception var9) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + var9, var9);
} finally {
ErrorContext.instance().reset();
}
return var5;
}
经Debug，发现在
var5 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
之前就已经完成了SQL语句的分页拼接，
具体怎样执行到SQL拼接的（即在何时调用了拦截器），还未找到。
PageHelper5.x的拦截器是：PageInterceptor类，它实现了Interceptor接口的intercept方法：
public Object intercept(Invocation invocation) throws Throwable {
Object var16;
try {
Object[] args = invocation.getArgs();
MappedStatement ms = (MappedStatement)args[0];
Object parameter = args[1];
RowBounds rowBounds = (RowBounds)args[2];
ResultHandler resultHandler = (ResultHandler)args[3];
Executor executor = (Executor)invocation.getTarget();
CacheKey cacheKey;
BoundSql boundSql;
if (args.length == 4) {
boundSql = ms.getBoundSql(parameter);
cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
} else {
cacheKey = (CacheKey)args[4];
boundSql = (BoundSql)args[5];
}
this.checkDialectExists();
List resultList;
if (!this.dialect.skip(ms, parameter, rowBounds)) {
if (this.dialect.beforeCount(ms, parameter, rowBounds)) {
Long count = this.count(executor, ms, parameter, rowBounds, resultHandler, boundSql);
if (!this.dialect.afterCount(count, parameter, rowBounds)) {
Object var12 = this.dialect.afterPage(new ArrayList(), parameter, rowBounds);
return var12;
}
}
resultList = ExecutorUtil.pageQuery(this.dialect, executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);
} else {
resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
}
var16 = this.dialect.afterPage(resultList, parameter, rowBounds);
} finally {
this.dialect.afterAll();
}
return var16;
}
进入ExecutorUtil.pageQuery：
public static <E> List<E> pageQuery(Dialect dialect, Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql, CacheKey cacheKey) throws SQLException {
if (!dialect.beforePage(ms, parameter, rowBounds)) {
return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
} else {
parameter = dialect.processParameterObject(ms, parameter, boundSql, cacheKey);
String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, cacheKey);
BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);
Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);
Iterator var12 = additionalParameters.keySet().iterator();
while(var12.hasNext()) {
String key = (String)var12.next();
pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
}
return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, pageBoundSql);
}
}
发现了获取分页SQL的代码：
String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, cacheKey);
进去看看，选择PageHelper的实现类：
public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {
return this.autoDialect.getDelegate().getPageSql(ms, boundSql, parameterObject, rowBounds, pageKey);
}
继续深入getPageSql：
public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {
String sql = boundSql.getSql();
Page page = this.getLocalPage();
String orderBy = page.getOrderBy();
if (StringUtil.isNotEmpty(orderBy)) {
pageKey.update(orderBy);
sql = OrderByParser.converToOrderBySql(sql, orderBy);
}
return page.isOrderByOnly() ? sql : this.getPageSql(sql, page, pageKey);
}
啊，终于找到你了：
Page page = this.getLocalPage();
看看getLocalPage()：
public <T> Page<T> getLocalPage() {
return PageHelper.getLocalPage();
}
继续进：
protected static final ThreadLocal<Page> LOCAL_PAGE = new ThreadLocal();
public static <T> Page<T> getLocalPage() {
return (Page)LOCAL_PAGE.get();
}
没错，就是他！在startPage中放进去的page！
所以，大致流程就是：
PageHelper将pageNum和pageSize放入ThreadLocal中，执行数据库前会从ThreadLocal中取出分页数据，然后拼接SQL，然后再执行SQL并返回结果。
Q.E.D.