模板模式

一、定义

在模板模式（Template Pattern）中，一个抽象类公开定义了执行它的方法的方式/模板。它的子类可以按需要重写方法实现，但调用将以抽象类中定义的方式进行。这种类型的设计模式属于行为型模式。模板方法模式实际上是封装一个固定流程，该流程由几个步骤组成，具体步骤可以由子类进行不同实现，从而让固定的流程产生不同的结果，它非常简单，其实就是类的继承机制，但它却是一个应用非常广泛的模式，模板方法模式的本质是抽象封装流程，具体进行实现。

模版方法模式中有两个主要的角色

• 抽象的模版方法 AbstractTemplate

1. 定义了一个或多个抽象操作，以便让子类实现。这些抽象操作叫做基本操作，它们是一个顶级逻辑的组成步骤。
2. 定义并实现了一个模板方法。这个模板方法一般是一个具体方法，它给出了一个顶级逻辑的骨架，而逻辑的组成步骤在相应的抽象操作中，推迟到子类实现。顶级逻辑也有可能调用一些具体方法。

• 具体的模版方法 ConcreteTemplate

1. 实现父类所定义的一个或多个抽象方法，它们是一个顶级逻辑的组成步骤。
2. 每一个抽象模板角色都可以有任意多个具体模板角色与之对应，而每一个具体模板角色都可以给出这些抽象方法（也就是顶级逻辑的组成步骤）的不同实现，从而使得顶级逻辑的实现各不相同。

 

 二、模板模式的案例

//抽象的模版类
public abstract  class AbstractTemplate {
    /**
     * 模版方法：顶层的统一实现逻辑，子类不能更改或者替换
     */
    public void templateMethod() {
        System.out.println("顶层模版类的模版方法");
        abstractMethod();
        hoodMethod();
        concreteMethod();
    }
    /**
     * 抽象方法：子类必须实现和替换的方法
     */
    public abstract void abstractMethod();

    /**
     * 钩子方法：子类可以选择性的实现或者不实现的方法
     */
    public void hoodMethod() {

    }

    /**
     * 基本方法：顶层模版类实现的方法，子类不能更改的方法
     */
    public void concreteMethod() {
        System.out.println("顶层模版类实现的具体方法");
    }
}

//具体的模版类
public class ConcreteTemplate  extends AbstractTemplate{

    @Override
    public void abstractMethod() {
        System.out.println("子类实现的抽象方法");
    }

    @Override
    public void hoodMethod() {
        // TODO Auto-generated method stub
        System.out.println("子类更改的钩子方法");
    }

}

public class TemplatePatternMain {
    public static void main(String[] args) {

        AbstractTemplate template = new ConcreteTemplate();

        template.templateMethod();
    }
}

三、模板模式在源码中的应用

下面代码是JDK 中的 AbstractList源码部分

package java.util;

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
    protected transient int modCount = 0;

    protected AbstractList() {
    }

    public boolean add(E var1) {
        this.add(this.size(), var1);
        return true;
    }

    public abstract E get(int var1);

从上面源码可以看到 get()是一个抽象方法，那么它的逻辑就是交给子类来实现，其实我们天天在用的ArrayList 就是 AbstractList 的子类。同理，有 AbstractList 就有 AbstractSet 和AbstractMap， 不信的朋友可以去看下对应的源码。现在上来都是封装的架构，现在的开发人员可能用的慢慢少了，以前很多程序员天天在用的HttpServlet，有三个方法 service()和 doGet()、doPost()方法，都是模板方法的抽象实现。看源码部分可以发现，其实在码源中这三个方法其实也没做啥子事，我们大都是覆盖了他原方法自己写处理逻辑。这个场景其实也可以叫成模板模式
 

public abstract class HttpServlet extends GenericServlet {
    private static final String METHOD_DELETE = "DELETE";
    private static final String METHOD_HEAD = "HEAD";
    private static final String METHOD_GET = "GET";
    private static final String METHOD_OPTIONS = "OPTIONS";
    private static final String METHOD_POST = "POST";
    private static final String METHOD_PUT = "PUT";
    private static final String METHOD_TRACE = "TRACE";
    private static final String HEADER_IFMODSINCE = "If-Modified-Since";
    private static final String HEADER_LASTMOD = "Last-Modified";
    private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings";
    private static ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.http.LocalStrings");

    public HttpServlet() {
    }

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_get_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

    protected long getLastModified(HttpServletRequest req) {
        return -1L;
    }

    protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        NoBodyResponse response = new NoBodyResponse(resp);
        this.doGet(req, response);
        response.setContentLength();
    }

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_post_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

    protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_put_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

今年来吃的没事干自己在学JDBC及MyBatis封装自己数据库架构插件，所以吃的没事又看了一下其中的源码，其实在MyBatis 框架也有一些好玩的东西，来看一下其中的 BaseExecutor 类，它是一个基础的SQL 执行类，实现了大部分的 SQL 执行逻辑，然后把几个方法交给子类定制化完成

public abstract class BaseExecutor implements Executor {
    private static final Log log = LogFactory.getLog(BaseExecutor.class);
    protected Transaction transaction;
    protected Executor wrapper;
    protected ConcurrentLinkedQueue<BaseExecutor.DeferredLoad> deferredLoads;
    protected PerpetualCache localCache;
    protected PerpetualCache localOutputParameterCache;
    protected Configuration configuration;
    protected int queryStack = 0;
    private boolean closed;

    protected BaseExecutor(Configuration configuration, Transaction transaction) {
        this.transaction = transaction;
        this.deferredLoads = new ConcurrentLinkedQueue();
        this.localCache = new PerpetualCache("LocalCache");
        this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
        this.closed = false;
        this.configuration = configuration;
        this.wrapper = this;
    }

    public Transaction getTransaction() {
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            return this.transaction;
        }
    }

    public void close(boolean forceRollback) {
        try {
            try {
                this.rollback(forceRollback);
            } finally {
                if (this.transaction != null) {
                    this.transaction.close();
                }

            }
        } catch (SQLException var11) {
            log.warn("Unexpected exception on closing transaction.  Cause: " + var11);
        } finally {
            this.transaction = null;
            this.deferredLoads = null;
            this.localCache = null;
            this.localOutputParameterCache = null;
            this.closed = true;
        }

    }

    public boolean isClosed() {
        return this.closed;
    }

    public int update(MappedStatement ms, Object parameter) throws SQLException {
        ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            this.clearLocalCache();
            return this.doUpdate(ms, parameter);
        }
    }

    public List<BatchResult> flushStatements() throws SQLException {
        return this.flushStatements(false);
    }

    public List<BatchResult> flushStatements(boolean isRollBack) throws SQLException {
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            return this.doFlushStatements(isRollBack);
        }
    }

    public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
        BoundSql boundSql = ms.getBoundSql(parameter);
        CacheKey key = this.createCacheKey(ms, parameter, rowBounds, boundSql);
        return this.query(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }

    public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
        ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            if (this.queryStack == 0 && ms.isFlushCacheRequired()) {
                this.clearLocalCache();
            }

            List list;
            try {
                ++this.queryStack;
                list = resultHandler == null ? (List)this.localCache.getObject(key) : null;
                if (list != null) {
                    this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
                } else {
                    list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
                }
            } finally {
                --this.queryStack;
            }

            if (this.queryStack == 0) {
                Iterator i$ = this.deferredLoads.iterator();

                while(i$.hasNext()) {
                    BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)i$.next();
                    deferredLoad.load();
                }

                this.deferredLoads.clear();
                if (this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
                    this.clearLocalCache();
                }
            }

            return list;
        }
    }

    public <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException {
        BoundSql boundSql = ms.getBoundSql(parameter);
        return this.doQueryCursor(ms, parameter, rowBounds, boundSql);
    }

    public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType) {
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            BaseExecutor.DeferredLoad deferredLoad = new BaseExecutor.DeferredLoad(resultObject, property, key, this.localCache, this.configuration, targetType);
            if (deferredLoad.canLoad()) {
                deferredLoad.load();
            } else {
                this.deferredLoads.add(new BaseExecutor.DeferredLoad(resultObject, property, key, this.localCache, this.configuration, targetType));
            }

        }
    }

    public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            CacheKey cacheKey = new CacheKey();
            cacheKey.update(ms.getId());
            cacheKey.update(rowBounds.getOffset());
            cacheKey.update(rowBounds.getLimit());
            cacheKey.update(boundSql.getSql());
            List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
            TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
            Iterator i$ = parameterMappings.iterator();

            while(i$.hasNext()) {
                ParameterMapping parameterMapping = (ParameterMapping)i$.next();
                if (parameterMapping.getMode() != ParameterMode.OUT) {
                    String propertyName = parameterMapping.getProperty();
                    Object value;
                    if (boundSql.hasAdditionalParameter(propertyName)) {
                        value = boundSql.getAdditionalParameter(propertyName);
                    } else if (parameterObject == null) {
                        value = null;
                    } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                        value = parameterObject;
                    } else {
                        MetaObject metaObject = this.configuration.newMetaObject(parameterObject);
                        value = metaObject.getValue(propertyName);
                    }

                    cacheKey.update(value);
                }
            }

            if (this.configuration.getEnvironment() != null) {
                cacheKey.update(this.configuration.getEnvironment().getId());
            }

            return cacheKey;
        }
    }

    public boolean isCached(MappedStatement ms, CacheKey key) {
        return this.localCache.getObject(key) != null;
    }

    public void commit(boolean required) throws SQLException {
        if (this.closed) {
            throw new ExecutorException("Cannot commit, transaction is already closed");
        } else {
            this.clearLocalCache();
            this.flushStatements();
            if (required) {
                this.transaction.commit();
            }

        }
    }

    public void rollback(boolean required) throws SQLException {
        if (!this.closed) {
            try {
                this.clearLocalCache();
                this.flushStatements(true);
            } finally {
                if (required) {
                    this.transaction.rollback();
                }

            }
        }

    }

    public void clearLocalCache() {
        if (!this.closed) {
            this.localCache.clear();
            this.localOutputParameterCache.clear();
        }

    }

    protected abstract int doUpdate(MappedStatement var1, Object var2) throws SQLException;

    protected abstract List<BatchResult> doFlushStatements(boolean var1) throws SQLException;

    protected abstract <E> List<E> doQuery(MappedStatement var1, Object var2, RowBounds var3, ResultHandler var4, BoundSql var5) throws SQLException;

    protected abstract <E> Cursor<E> doQueryCursor(MappedStatement var1, Object var2, RowBounds var3, BoundSql var4) throws SQLException;

    protected void closeStatement(Statement statement) {
        if (statement != null) {
            try {
                statement.close();
            } catch (SQLException var3) {
            }
        }

    }

    protected void applyTransactionTimeout(Statement statement) throws SQLException {
        StatementUtil.applyTransactionTimeout(statement, statement.getQueryTimeout(), this.transaction.getTimeout());
    }

    private void handleLocallyCachedOutputParameters(MappedStatement ms, CacheKey key, Object parameter, BoundSql boundSql) {
        if (ms.getStatementType() == StatementType.CALLABLE) {
            Object cachedParameter = this.localOutputParameterCache.getObject(key);
            if (cachedParameter != null && parameter != null) {
                MetaObject metaCachedParameter = this.configuration.newMetaObject(cachedParameter);
                MetaObject metaParameter = this.configuration.newMetaObject(parameter);
                Iterator i$ = boundSql.getParameterMappings().iterator();

                while(i$.hasNext()) {
                    ParameterMapping parameterMapping = (ParameterMapping)i$.next();
                    if (parameterMapping.getMode() != ParameterMode.IN) {
                        String parameterName = parameterMapping.getProperty();
                        Object cachedValue = metaCachedParameter.getValue(parameterName);
                        metaParameter.setValue(parameterName, cachedValue);
                    }
                }
            }
        }

    }

    private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
        this.localCache.putObject(key, ExecutionPlaceholder.EXECUTION_PLACEHOLDER);

        List list;
        try {
            list = this.doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
        } finally {
            this.localCache.removeObject(key);
        }

        this.localCache.putObject(key, list);
        if (ms.getStatementType() == StatementType.CALLABLE) {
            this.localOutputParameterCache.putObject(key, parameter);
        }

        return list;
    }

    protected Connection getConnection(Log statementLog) throws SQLException {
        Connection connection = this.transaction.getConnection();
        return statementLog.isDebugEnabled() ? ConnectionLogger.newInstance(connection, statementLog, this.queryStack) : connection;
    }

    public void setExecutorWrapper(Executor wrapper) {
        this.wrapper = wrapper;
    }

    private static class DeferredLoad {
        private final MetaObject resultObject;
        private final String property;
        private final Class<?> targetType;
        private final CacheKey key;
        private final PerpetualCache localCache;
        private final ObjectFactory objectFactory;
        private final ResultExtractor resultExtractor;

        public DeferredLoad(MetaObject resultObject, String property, CacheKey key, PerpetualCache localCache, Configuration configuration, Class<?> targetType) {
            this.resultObject = resultObject;
            this.property = property;
            this.key = key;
            this.localCache = localCache;
            this.objectFactory = configuration.getObjectFactory();
            this.resultExtractor = new ResultExtractor(configuration, this.objectFactory);
            this.targetType = targetType;
        }

        public boolean canLoad() {
            return this.localCache.getObject(this.key) != null && this.localCache.getObject(this.key) != ExecutionPlaceholder.EXECUTION_PLACEHOLDER;
        }

        public void load() {
            List<Object> list = (List)this.localCache.getObject(this.key);
            Object value = this.resultExtractor.extractObjectFromList(list, this.targetType);
            this.resultObject.setValue(this.property, value);
        }
    }
}

上面的源码中如 doUpdate、doFlushStatements、doQuery、doQueryCursor 这几个方法就是交给了它的子类去实现了

从上面类图可以看到他有四个子类，随便找两个子类的实现对比下

 public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
        Statement stmt = null;

        int var6;
        try {
            Configuration configuration = ms.getConfiguration();
            StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, (ResultHandler)null, (BoundSql)null);
            stmt = this.prepareStatement(handler, ms.getStatementLog());
            var6 = handler.update(stmt);
        } finally {
            this.closeStatement(stmt);
        }

        return var6;
    }

public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, (ResultHandler)null, (BoundSql)null);
        BoundSql boundSql = handler.getBoundSql();
        String sql = boundSql.getSql();
        Statement stmt;
        if (sql.equals(this.currentSql) && ms.equals(this.currentStatement)) {
            int last = this.statementList.size() - 1;
            stmt = (Statement)this.statementList.get(last);
            this.applyTransactionTimeout(stmt);
            handler.parameterize(stmt);
            BatchResult batchResult = (BatchResult)this.batchResultList.get(last);
            batchResult.addParameterObject(parameterObject);
        } else {
            Connection connection = this.getConnection(ms.getStatementLog());
            stmt = handler.prepare(connection, this.transaction.getTimeout());
            handler.parameterize(stmt);
            this.currentSql = sql;
            this.currentStatement = ms;
            this.statementList.add(stmt);
            this.batchResultList.add(new BatchResult(ms, sql, parameterObject));
        }

        handler.batch(stmt);
        return -2147482646;
    }

从上面可以看到SimpleExecutor是一种常规执行器，每次执行都会创建一个statement，用完后关闭，而BatchExecutor是批处理型执行器，doUpdate预处理存储过程或批处理操作

四、总结

优点：
1、利用模板方法将相同处理逻辑的代码放到抽象父类中，可以提高代码的复用性。
2、将不同的代码不同的子类中，通过对子类的扩展增加新的行为，提高代码的扩展性。
3、把不变的行为写在父类上，去除子类的重复代码，提供了一个很好的代码复用平台，符合开闭原则。

缺点：
1、类数目的增加，每一个抽象类都需要一个子类来实现，这样导致类的个数增加。
2、类数量的增加，间接地增加了系统实现的复杂度。
3、继承关系自身缺点，如果父类添加新的抽象方法，所有子类都要改一遍。

补充：这种玩意其实在算法场景中应用的比较多，或者是在共性多的场景也可以这么玩

git:源码：https://github.com/ljx958720/design_patterns.git