在Mybatis中插件最經常使用的是作為分頁插件，接下來我們通過實作Interceptor來完成一個分頁插件。
雖然Mybatis也提供了分頁操作，通過在sqlSession的接口函數中設定RowBounds，給RowBounds設定初值（RowBounds源碼）來實作邏輯分頁，其實作原理就是通過sql查詢所有的結果，并将結果放到List中，然後根據RowBouds的limit和offset數值來傳回最後的資料，這種邏輯分頁在資料量比較大的情況下對性能是有影響的。雖然我們可以自己寫帶有分頁語句的sql來實作實體分頁，如mysql下：select * from table limit 10 offset 1，來獲得分頁結果，但不同的資料庫産品（mysql、oracle、sqlserver和oracle）對應的分頁語句并不相同，如果要同時相容所有的資料庫産品，需要開發人員在每個分頁sql中都編碼幾種資料庫産品的分頁語句，這樣的開發效率實在是太低了，Mybatis的插件Interceptor給了我們一種根據資料庫類型自動組裝sql語句的方法。
PageInterceptor源碼如下：
package com.tianjunwei.page;
import java.lang.reflect.Constructor;
import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.util.List;
import java.util.Properties;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.ParameterMapping;
import org.apache.ibatis.mapping.SqlSource;
import org.apache.ibatis.plugin.Interceptor;
import org.apache.ibatis.plugin.Intercepts;
import org.apache.ibatis.plugin.Invocation;
import org.apache.ibatis.plugin.Plugin;
import org.apache.ibatis.plugin.Signature;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.mapping.MappedStatement.Builder;
import com.tianjunwei.page.dialect.Dialect;
import com.tianjunwei.page.dialect.DialectFactory;
/*
* 分頁插件我們隻需要攔截Executor的query方法即可，在執行sql語句之前組裝新的分頁sql語句
*/
@Intercepts({@Signature(
type= Executor.class,
method = "query",
args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})
public class PageInterceptor implements Interceptor{
String dialectClass;
boolean asyncTotalCount = false;
String dataBaseType=null;
public static ThreadLocal<RowBounds> PageRowBounds = new ThreadLocal<RowBounds>();
@SuppressWarnings({"rawtypes", "unchecked"})
public Object intercept(final Invocation invocation) throws Throwable {
//Executor的實作類
final Executor executor = (Executor) invocation.getTarget();
//Executor的query函數的參數
final Object[] queryArgs = invocation.getArgs();
final MappedStatement ms = (MappedStatement)queryArgs[0];
final Object parameter = queryArgs[1];
//rowBounds中有分頁語句的limit和offset值
RowBounds rowBounds = (RowBounds)queryArgs[2];
if((PageRowBounds.get() != null)&&(PageRowBounds.get().getLimit() != RowBounds.NO_ROW_LIMIT || PageRowBounds.get().getOffset() != RowBounds.NO_ROW_OFFSET)){
rowBounds = PageRowBounds.get();
}
//如果不需要分頁操作，直接傳回，rowBounds為預設值時
if(rowBounds.getOffset() == RowBounds.NO_ROW_OFFSET
&& rowBounds.getLimit() == RowBounds.NO_ROW_LIMIT){
return invocation.proceed();
}
//根據不同的資料庫擷取不到的分頁方言來
if(dialectClass == null || "".equals(dialectClass)){
//判斷資料源選擇方言，暫時支援mysql、oracle、postgresql和sql server 2005 2008及2012
Connection connection = executor.getTransaction().getConnection();
DatabaseMetaData databaseMetaData = null;
if(connection != null){
databaseMetaData = connection.getMetaData();
}else {
throw new Exception("connection is null");
}
String databaseProductName = databaseMetaData.getDatabaseProductName();
if( dataBaseType == null || "".equals(dataBaseType)){
dataBaseType = databaseProductName;
}
//通過xml方言的配置來獲得方言類
if(databaseProductName != null && !("".equals(dataBaseType))){
dialectClass = DialectFactory.getDialectClass(dataBaseType,databaseProductName);
}else{
throw new Exception("the property of dialect is null");
}
setDialectClass(dialectClass);
}
final Dialect dialect;
try {
//初始化分頁方言類
Class clazz = Class.forName(dialectClass);
Constructor constructor = clazz.getConstructor(new Class[]{MappedStatement.class, Object.class, RowBounds.class});
dialect = (Dialect)constructor.newInstance(new Object[]{ms, parameter, rowBounds});
} catch (Exception e) {
throw new ClassNotFoundException("Cannot create dialect instance: "+dialectClass,e);
}
final BoundSql boundSql = ms.getBoundSql(parameter);
//建立新的MappedStatement，此時的sql語句已經是符合資料庫産品的分頁語句
//dialect.getPageSQL()獲得分頁語句
//dialect.getParameterMappings(), dialect.getParameterObject(),添加了兩個參數及其值，兩個參數為_limit和_offset
queryArgs[0] = copyFromNewSql(ms,boundSql,dialect.getPageSQL(), dialect.getParameterMappings(), dialect.getParameterObject());
//sql語句的參數集合
queryArgs[1] = dialect.getParameterObject();
//設定為不分頁，由新的sql語句進行實體分頁
queryArgs[2] = new RowBounds(RowBounds.NO_ROW_OFFSET,RowBounds.NO_ROW_LIMIT);
return invocation.proceed();
}
//這個方法是用于mybatis接口程式設計過程中顯示的指定分頁參數
public static void setPage(int pageNumber,int pageSize){
RowBounds pageRowBounds = null;
if(pageNumber > 0)
pageRowBounds = new RowBounds((pageNumber-1)*pageSize, pageSize);
else {
pageRowBounds = new RowBounds(0, pageSize);
}
PageRowBounds.set(pageRowBounds);
}
//建立新的MappedStatement
private MappedStatement copyFromNewSql(MappedStatement ms, BoundSql boundSql,
String sql, List<ParameterMapping> parameterMappings, Object parameter){
//根據新的分頁sql語句建立BoundSql
BoundSql newBoundSql = copyFromBoundSql(ms, boundSql, sql, parameterMappings, parameter);
//根據newBoundSql建立新的MappedStatement
return copyFromMappedStatement(ms, new BoundSqlSqlSource(newBoundSql));
}
//根據新的分頁sql語句建立BoundSql
private BoundSql copyFromBoundSql(MappedStatement ms, BoundSql boundSql,
String sql, List<ParameterMapping> parameterMappings,Object parameter) {
BoundSql newBoundSql = new BoundSql(ms.getConfiguration(),sql, parameterMappings, parameter);
for (ParameterMapping mapping : boundSql.getParameterMappings()) {
String prop = mapping.getProperty();
if (boundSql.hasAdditionalParameter(prop)) {
newBoundSql.setAdditionalParameter(prop, boundSql.getAdditionalParameter(prop));
}
}
return newBoundSql;
}
//根據newBoundSql建立新的MappedStatement
private MappedStatement copyFromMappedStatement(MappedStatement ms,SqlSource newSqlSource) {
Builder builder = new Builder(ms.getConfiguration(),ms.getId(),newSqlSource,ms.getSqlCommandType());
builder.resource(ms.getResource());
builder.fetchSize(ms.getFetchSize());
builder.statementType(ms.getStatementType());
builder.keyGenerator(ms.getKeyGenerator());
if(ms.getKeyProperties() != null && ms.getKeyProperties().length !=0){
StringBuffer keyProperties = new StringBuffer();
for(String keyProperty : ms.getKeyProperties()){
keyProperties.append(keyProperty).append(",");
}
keyProperties.delete(keyProperties.length()-1, keyProperties.length());
builder.keyProperty(keyProperties.toString());
}
//setStatementTimeout()
builder.timeout(ms.getTimeout());
//setStatementResultMap()
builder.parameterMap(ms.getParameterMap());
//setStatementResultMap()
builder.resultMaps(ms.getResultMaps());
builder.resultSetType(ms.getResultSetType());
//setStatementCache()
builder.cache(ms.getCache());
builder.flushCacheRequired(ms.isFlushCacheRequired());
builder.useCache(ms.isUseCache());
return builder.build();
}
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
/**
* @Title: setProperties
* @Description: 方言插件配置時設定的參數
* @param properties 參數
* @return void
* @2016年1月13日下午3:54:47
*/
public void setProperties(Properties properties) {
dataBaseType = properties.getProperty("dialectType");
}
public static class BoundSqlSqlSource implements SqlSource {
BoundSql boundSql;
public BoundSqlSqlSource(BoundSql boundSql) {
this.boundSql = boundSql;
}
public BoundSql getBoundSql(Object parameterObject) {
return boundSql;
}
}
public void setDialectClass(String dialectClass) {
this.dialectClass = dialectClass;
}
public void setDataBaseType(String dataBaseType) {
this.dataBaseType = dataBaseType;
}
} ![龍珠訓練營task04[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)