面试官:Mybatis是如何运用设计模式的?
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Builder模式,例如SqlSessionFactoryBuilder、XMLConfigBuilder、XMLMapperBuilder、XMLStatementBuilder、CacheBuilder;
工厂模式,例如SqlSessionFactory、ObjectFactory、MapperProxyFactory;
单例模式,例如ErrorContext和LogFactory;
代理模式,Mybatis实现的核心,比如MapperProxy、ConnectionLogger,用的jdk的动态代理;还有executor.loader包使用了cglib或者javassist达到延迟加载的效果;
组合模式,例如SqlNode和各个子类ChooseSqlNode等;
模板方法模式,例如BaseExecutor和SimpleExecutor,还有BaseTypeHandler和所有的子类例如IntegerTypeHandler;
适配器模式,例如Log的Mybatis接口和它对jdbc、log4j等各种日志框架的适配实现;
装饰者模式,例如Cache包中的cache.decorators子包中等各个装饰者的实现;
迭代器模式,例如迭代器模式PropertyTokenizer;
# Builder模式
# 工厂模式
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level,
boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call
// close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
public final class LogFactory {
private static Constructor<? extends Log> logConstructor;
private LogFactory() {
// disable construction
}
public static Log getLog(Class<?> aClass) {
return getLog(aClass.getName());
}
# 单例模式
public class ErrorContext {
private static final ThreadLocal<ErrorContext> LOCAL = new ThreadLocal<ErrorContext>();
private ErrorContext() {
}
public static ErrorContext instance() {
ErrorContext context = LOCAL.get();
if (context == null) {
context = new ErrorContext();
LOCAL.set(context);
}
return context;
}
# 代理模式
Subject: 抽象主题角色
Proxy: 代理主题角色
RealSubject: 真实主题角色
/**
* @author Lasse Voss
*/
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class<T> getMapperInterface() {
return mapperInterface;
}
public Map<Method, MapperMethod> getMethodCache() {
return methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface },
mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}
public class MapperProxy<T> implements InvocationHandler, Serializable {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
# 组合模式
<update id="update" parameterType="org.format.dynamicproxy.mybatis.bean.User">
UPDATE users
<trim prefix="SET" prefixOverrides=",">
<if test="name != null and name != ''">
name = #{name}
</if>
<if test="age != null and age != ''">
, age = #{age}
</if>
<if test="birthday != null and birthday != ''">
, birthday = #{birthday}
</if>
</trim>
where id = ${id}
</update>
public interface SqlNode {
boolean apply(DynamicContext context);
}
@Override
public boolean apply(DynamicContext context) {
GenericTokenParser parser = createParser(new BindingTokenParser(context, injectionFilter));
context.appendSql(parser.parse(text));
return true;
}
@Override
public boolean apply(DynamicContext context) {
if (evaluator.evaluateBoolean(test, context.getBindings())) {
contents.apply(context);
return true;
}
return false;
}
# 模板方法模式
protected abstract int doUpdate(MappedStatement ms, Object parameter) throws SQLException;
protected abstract List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException;
protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds,
ResultHandler resultHandler, BoundSql boundSql) throws SQLException;
简单SimpleExecutor:每执行一次update或select,就开启一个Statement对象,用完立刻关闭Statement对象。(可以是Statement或PrepareStatement对象)
重用ReuseExecutor:执行update或select,以sql作为key查找Statement对象,存在就使用,不存在就创建,用完后,不关闭Statement对象,而是放置于Map
内,供下一次使用。(可以是Statement或PrepareStatement对象)
批量BatchExecutor:执行update(没有select,JDBC批处理不支持select),将所有sql都添加到批处理中(addBatch()),等待统一执行(executeBatch()),它缓存了多个Statement对象,每个Statement对象都是addBatch()完毕后,等待逐一执行executeBatch()批处理的;BatchExecutor相当于维护了多个桶,每个桶里都装了很多属于自己的SQL,就像苹果蓝里装了很多苹果,番茄蓝里装了很多番茄,最后,再统一倒进仓库。(可以是Statement或PrepareStatement对象)
@Override
public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null,
null);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.update(stmt);
} finally {
closeStatement(stmt);
}
}
# 适配器模式
/**
* @author Clinton Begin
*/
public interface Log {
boolean isDebugEnabled();
boolean isTraceEnabled();
void error(String s, Throwable e);
void error(String s);
void debug(String s);
void trace(String s);
void warn(String s);
}
public class Log4jImpl implements Log {
private static final String FQCN = Log4jImpl.class.getName();
private Logger log;
public Log4jImpl(String clazz) {
log = Logger.getLogger(clazz);
}
@Override
public boolean isDebugEnabled() {
return log.isDebugEnabled();
}
@Override
public boolean isTraceEnabled() {
return log.isTraceEnabled();
}
@Override
public void error(String s, Throwable e) {
log.log(FQCN, Level.ERROR, s, e);
}
@Override
public void error(String s) {
log.log(FQCN, Level.ERROR, s, null);
}
@Override
public void debug(String s) {
log.log(FQCN, Level.DEBUG, s, null);
}
@Override
public void trace(String s) {
log.log(FQCN, Level.TRACE, s, null);
}
@Override
public void warn(String s) {
log.log(FQCN, Level.WARN, s, null);
}
}
# 装饰者模式
FifoCache:先进先出算法,缓存回收策略
LoggingCache:输出缓存命中的日志信息
LruCache:最近最少使用算法,缓存回收策略
ScheduledCache:调度缓存,负责定时清空缓存
SerializedCache:缓存序列化和反序列化存储
SoftCache:基于软引用实现的缓存管理策略
SynchronizedCache:同步的缓存装饰器,用于防止多线程并发访问
WeakCache:基于弱引用实现的缓存管理策略
另外,还有一个特殊的装饰器TransactionalCache:事务性的缓存
一级缓存,又叫本地缓存,是PerpetualCache类型的永久缓存,保存在执行器中(BaseExecutor),而执行器又在SqlSession(DefaultSqlSession)中,所以一级缓存的生命周期与SqlSession是相同的。
二级缓存,又叫自定义缓存,实现了Cache接口的类都可以作为二级缓存,所以可配置如encache等的第三方缓存。二级缓存以namespace名称空间为其唯一标识,被保存在Configuration核心配置对象中。
SynchronizedCache–>LoggingCache–>SerializedCache–>ScheduledCache–>LruCache–>PerpetualCache
# 迭代器模式
public class PropertyTokenizer implements Iterator<PropertyTokenizer> {
private String name;
private String indexedName;
private String index;
private String children;
public PropertyTokenizer(String fullname) {
int delim = fullname.indexOf('.');
if (delim > -1) {
name = fullname.substring(0, delim);
children = fullname.substring(delim + 1);
} else {
name = fullname;
children = null;
}
indexedName = name;
delim = name.indexOf('[');
if (delim > -1) {
index = name.substring(delim + 1, name.length() - 1);
name = name.substring(0, delim);
}
}
public String getName() {
return name;
}
public String getIndex() {
return index;
}
public String getIndexedName() {
return indexedName;
}
public String getChildren() {
return children;
}
@Override
public boolean hasNext() {
return children != null;
}
@Override
public PropertyTokenizer next() {
return new PropertyTokenizer(children);
}
@Override
public void remove() {
throw new UnsupportedOperationException(
"Remove is not supported, as it has no meaning in the context of properties.");
}
}
参考资料
图说设计模式
http://design-patterns.readthedocs.io/zh_CN/latest/index.html深入浅出Mybatis系列(十)—SQL执行流程分析(源码篇)
http://www.cnblogs.com/dongying/p/4142476.html设计模式读书笔记—–组合模式
http://www.cnblogs.com/chenssy/p/3299719.htmlMybatis3.3.x技术内幕(四):五鼠闹东京之执行器Executor设计原本
http://blog.csdn.net/wagcy/article/details/32963235mybatis缓存机制详解(一)——Cache
https://my.oschina.net/lixin91/blog/620068
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