MyBatis 示例-插件

簡介

利用 MyBatis Plugin 插件技術實現分頁功能。

分頁插件實現思路如下：

業務代碼在 ThreadLocal 中保存分頁信息；
MyBatis Interceptor 攔截查詢請求，獲取分頁信息，實現分頁操作，封裝分頁列表數據返回；

測試類：com.yjw.demo.PageTest

插件開發過程

確定需要攔截的簽名

MyBatis 插件可以攔截四大對象中的任意一個，從 Plugin 源碼中可以看到它需要註冊簽名才能夠運行插件，簽名需要確定一些要素。

確定需要攔截的對象

Executor 是執行 SQL 的全過程，包括組裝參數，組裝結果集返回和執行 SQL 過程，都可以攔截。
StatementHandler 是執行 SQL 的過程，我們可以重寫執行 SQL 的過程。
ParameterHandler 是攔截執行 SQL 的參數組裝，我們可以重寫組裝參數規則。
ResultSetHandler 用於攔截執行結果的組裝，我們可以重寫組裝結果的規則。

攔截方法和參數

當確定了需要攔截什麼對象，接下來就要確定需要攔截什麼方法和方法的參數。比如分頁插件需要攔截 Executor 的 query 方法，我們先看看 Executor 接口的定義，代碼清單如下：

public interface Executor {
 
  ResultHandler NO_RESULT_HANDLER = null;
 
  int update(MappedStatement ms, Object parameter) throws SQLException;
 
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
 
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
 
  <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
 
  List<BatchResult> flushStatements() throws SQLException;
 
  void commit(boolean required) throws SQLException;
 
  void rollback(boolean required) throws SQLException;
 
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
 
  boolean isCached(MappedStatement ms, CacheKey key);
 
  void clearLocalCache();
 
  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
 
  Transaction getTransaction();
 
  void close(boolean forceRollback);
 
  boolean isClosed();
 
  void setExecutorWrapper(Executor executor);
 
}

以上的任何方法都可以攔截，從接口定義而言，query 方法有兩個，我們可以按照代碼清單來定義簽名。

@Intercepts({
        @Signature(type = Executor.class, method = "query",
                args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
        @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class,
                ResultHandler.class, CacheKey.class, BoundSql.class})})

其中，@Intercepts 說明它是一個攔截器。@Signature 是註冊攔截器簽名的地方，type 是四大對象中的一個，method 是需要攔截的方法，args 是方法的參數。

插件接口定義

在 MyBatis 中開發插件，需要實現 Interceptor 接口，接口的定義如下：

public interface Interceptor {
 
  Object intercept(Invocation invocation) throws Throwable;
 
  Object plugin(Object target);
 
  void setProperties(Properties properties);
 
}

intercept 方法：它將直接覆蓋你所攔截對象原有的方法，因此它是插件的核心方法。通過 invocation 參數可以反射調度原來對象的方法。
plugin 方法：target 是被攔截對象，它的作用是給被攔截對象生成一個代理對象，並返回它。爲了方便 MyBatis 使用 org.apache.ibatis.plugin.Plugin 中的 wrap 靜態方法提供生成代理對象。
setProperties 方法：允許在 plugin 元素中配置所需參數，方法在插件初始化的時候就被調用了一次，然後把插件對象存入到配置中，以便後面再取出。

實現類

根據分頁插件的實現思路，定義了三個類。

Page 類

Page 類繼承了 ArrayList 類，用來封裝分頁信息和列表數據。

/**
 * 分頁返回對象
 * 
 * @author yinjianwei
 * @date 2018/11/05
 */
public class Page<E> extends ArrayList<E> {

    private static final long serialVersionUID = 1L;

    /**
     * 頁碼，從1開始
     */
    private int pageNum;
    /**
     * 頁面大小
     */
    private int pageSize;
    /**
     * 起始行
     */
    private int startRow;
    /**
     * 末行
     */
    private int endRow;
    /**
     * 總數
     */
    private long total;
    /**
     * 總頁數
     */
    private int pages;

    public int getPageNum() {
        return pageNum;
    }

    public void setPageNum(int pageNum) {
        this.pageNum = pageNum;
    }

    public int getPageSize() {
        return pageSize;
    }

    public void setPageSize(int pageSize) {
        this.pageSize = pageSize;
    }

    public int getStartRow() {
        return startRow;
    }

    public void setStartRow(int startRow) {
        this.startRow = startRow;
    }

    public int getEndRow() {
        return endRow;
    }

    public void setEndRow(int endRow) {
        this.endRow = endRow;
    }

    public long getTotal() {
        return total;
    }

    public void setTotal(long total) {
        this.total = total;
        this.pages = (int)(total / pageSize + (total % pageSize == 0 ? 0 : 1));
        if (pageNum > pages) {
            pageNum = pages;
        }
        this.startRow = this.pageNum > 0 ? (this.pageNum - 1) * this.pageSize : 0;
        this.endRow = this.startRow + this.pageSize * (this.pageNum > 0 ? 1 : 0);
    }

    public int getPages() {
        return pages;
    }

    public void setPages(int pages) {
        this.pages = pages;
    }

    /**
     * 返回當前對象
     * 
     * @return
     */
    public List<E> getResult() {
        return this;
    }

}

PageHelper 類

PageHelper 類是分頁的幫助類，主要利用 ThreadLocal 線程變量存儲分頁信息。代碼清單如下：

/**
 * 分頁幫助類
 * 
 * @author yinjianwei
 * @date 2018/11/05
 */
@SuppressWarnings("rawtypes")
public class PageHelper {

    private static final ThreadLocal<Page> PAGE_THREADLOCAT = new ThreadLocal<Page>();

    /**
     * 設置線程局部變量分頁信息
     * 
     * @param page
     */
    public static void setPageThreadLocal(Page page) {
        PAGE_THREADLOCAT.set(page);
    }

    /**
     * 獲取線程局部變量分頁信息
     * 
     * @return
     */
    public static Page getPageThreadLocal() {
        return PAGE_THREADLOCAT.get();
    }

    /**
     * 清空線程局部變量分頁信息
     */
    public static void pageThreadLocalClear() {
        PAGE_THREADLOCAT.remove();
    }

    /**
     * 設置分頁參數
     * 
     * @param pageNum
     * @param pageSize
     */
    public static void startPage(Integer pageNum, Integer pageSize) {
        Page page = new Page();
        page.setPageNum(pageNum);
        page.setPageSize(pageSize);
        setPageThreadLocal(page);
    }

}

PageInterceptor 類

PageInterceptor 類實現了 Interceptor 接口，是分頁插件的核心類。代碼清單如下：

/**
 * 分頁攔截器
 * 
 * @author yinjianwei
 * @date 2018/11/05
 */
@Intercepts({
    @Signature(type = Executor.class, method = "query",
        args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
    @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class,
        ResultHandler.class, CacheKey.class, BoundSql.class})})
public class PageInterceptor implements Interceptor {

    private Field additionalParametersField;

    @SuppressWarnings({"rawtypes", "unchecked"})
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        Executor executor = (Executor)invocation.getTarget();
        Object[] args = invocation.getArgs();
        MappedStatement ms = (MappedStatement)args[0];
        Object parameter = args[1];
        RowBounds rowBounds = (RowBounds)args[2];
        ResultHandler resultHandler = (ResultHandler)args[3];
        CacheKey cacheKey;
        BoundSql boundSql;
        // 4個參數
        if (args.length == 4) {
            boundSql = ms.getBoundSql(parameter);
            cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
        }
        // 6個參數
        else {
            cacheKey = (CacheKey)args[4];
            boundSql = (BoundSql)args[5];
        }
        // 判斷是否需要分頁
        Page page = PageHelper.getPageThreadLocal();
        // 不執行分頁
        if (page.getPageNum() <= 0) {
            return executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
        }
        // count查詢
        MappedStatement countMs = newCountMappedStatement(ms);
        String sql = boundSql.getSql();
        String countSql = "select count(1) from (" + sql + ") _count";
        BoundSql countBoundSql =
            new BoundSql(ms.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter);
        Map<String, Object> additionalParameters = (Map<String, Object>)additionalParametersField.get(boundSql);
        for (Entry<String, Object> additionalParameter : additionalParameters.entrySet()) {
            countBoundSql.setAdditionalParameter(additionalParameter.getKey(), additionalParameter.getValue());
        }
        CacheKey countCacheKey = executor.createCacheKey(countMs, parameter, rowBounds, countBoundSql);
        Object countResult =
            executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countCacheKey, countBoundSql);
        Long count = (Long)((List)countResult).get(0);
        page.setTotal(count);
        // 分頁查詢
        String pageSql = sql + " limit " + page.getStartRow() + "," + page.getPageSize();
        BoundSql pageBoundSql =
            new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);
        for (Entry<String, Object> additionalParameter : additionalParameters.entrySet()) {
            pageBoundSql.setAdditionalParameter(additionalParameter.getKey(), additionalParameter.getValue());
        }
        CacheKey pageCacheKey = executor.createCacheKey(ms, parameter, rowBounds, pageBoundSql);
        List listResult = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageCacheKey, pageBoundSql);
        page.addAll(listResult);
        // 清空線程局部變量分頁信息
        PageHelper.pageThreadLocalClear();
        return page;
    }

    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }

    @Override
    public void setProperties(Properties properties) {
        try {
            additionalParametersField = BoundSql.class.getDeclaredField("additionalParameters");
            additionalParametersField.setAccessible(true);
        } catch (NoSuchFieldException | SecurityException e) {
            e.printStackTrace();
        }
    }

    /**
     * 創建count的MappedStatement
     * 
     * @param ms
     * @return
     */
    private MappedStatement newCountMappedStatement(MappedStatement ms) {
        MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), ms.getId() + "_count",
            ms.getSqlSource(), 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) {
            StringBuilder keyProperties = new StringBuilder();
            for (String keyProperty : ms.getKeyProperties()) {
                keyProperties.append(keyProperty).append(",");
            }
            keyProperties.delete(keyProperties.length() - 1, keyProperties.length());
            builder.keyProperty(keyProperties.toString());
        }
        builder.timeout(ms.getTimeout());
        builder.parameterMap(ms.getParameterMap());
        // count查詢返回值int
        List<ResultMap> resultMaps = new ArrayList<ResultMap>();
        ResultMap resultMap = new ResultMap.Builder(ms.getConfiguration(), ms.getId() + "_count", Long.class,
            new ArrayList<ResultMapping>(0)).build();
        resultMaps.add(resultMap);
        builder.resultMaps(resultMaps);
        builder.resultSetType(ms.getResultSetType());
        builder.cache(ms.getCache());
        builder.flushCacheRequired(ms.isFlushCacheRequired());
        builder.useCache(ms.isUseCache());

        return builder.build();
    }

}

配置

MyBatis 配置文件增加 plugin 配置項。

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

    <typeHandlers>
        <typeHandler javaType="com.yjw.demo.mybatis.common.constant.Sex"
                     jdbcType="TINYINT"
                     handler="com.yjw.demo.mybatis.common.type.SexEnumTypeHandler"/>
    </typeHandlers>

    <plugins>
        <plugin interceptor="com.yjw.demo.mybatis.common.page.PageInterceptor">
        </plugin>
    </plugins>
</configuration>