MyBatis的plugins解析

MyBatis 的 plugins 解析

插件

MyBatis 允許你在已映射語句執行過程中的某一點進行攔截調用。默認情況下，MyBatis 允許使用插件來攔截的方法調用包括：

• Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
• ParameterHandler (getParameterObject, setParameters)
• ResultSetHandler (handleResultSets, handleOutputParameters)
• StatementHandler (prepare, parameterize, batch, update, query)

其配置方式如下

<plugins>
    <plugin interceptor="com.yczuoxin.interceptor.SQLInterceptor">
        <property name="zuoxin" value="sql" />
    </plugin>
    <plugin interceptor="com.yczuoxin.interceptor.MyPageInterceptor" />
</plugins>

Interceptor

public interface Interceptor {
	// 把增強的方法寫在這個這個方法的實現類中
    Object intercept(Invocation invocation) throws Throwable;
	// interceptorChian 會默認調用這個方法將我們要代理的對象進行代理
    default Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }

    default void setProperties(Properties properties) {
        // NOP
    }

}

Invocation

public class Invocation {

    private final Object target;
    private final Method method;
    private final Object[] args;

    public Invocation(Object target, Method method, Object[] args) {
        this.target = target;
        this.method = method;
        this.args = args;
    }
	// 被代理對象
    public Object getTarget() {
        return target;
    }
	// 被代理方法
    public Method getMethod() {
        return method;
    }
	// 被代理方法的參數
    public Object[] getArgs() {
        return args;
    }
	// 調用被代理對象的被代理方法
    public Object proceed() throws InvocationTargetException, IllegalAccessException {
        return method.invoke(target, args);
    }

}

@Intercepts

這個註解放在攔截器插件類上來標明這個插件攔截的是哪個類上的哪個方法

• @Intercepts
  • @Signature[]
    • type 哪個類
    • method 哪個方法
    • args[] 入參是什麼

InterceptorChain

Mybatis 利用這個對象將插件保存在一個 ArrayList 中

public class InterceptorChain {

    private final List<Interceptor> interceptors = new ArrayList<>();
	
    // 此處可以看出被代理過的對象還可以再次被代理
    public Object pluginAll(Object target) {
        for (Interceptor interceptor : interceptors) {
            target = interceptor.plugin(target);
        }
        return target;
    }

    public void addInterceptor(Interceptor interceptor) {
        interceptors.add(interceptor);
    }

    public List<Interceptor> getInterceptors() {
        return Collections.unmodifiableList(interceptors);
    }

}

看名字就知道這個類是採用的責任鏈模式處理插件的，並且使用的是攔截器而不是過濾器的方式，所以代理模式應該是實現插件必不可少的

Plugin

此時我們再看下 Plugin 類就知道 MyBatis 是用什麼方式實現插件功能了

public class Plugin implements InvocationHandler {

    private final Object target;
    private final Interceptor interceptor;
    private final Map<Class<?>, Set<Method>> signatureMap;

    private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
        this.target = target;
        this.interceptor = interceptor;
        this.signatureMap = signatureMap;
    }

    public static Object wrap(Object target, Interceptor interceptor) {
        Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
        Class<?> type = target.getClass();
        Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
        if (interfaces.length > 0) {
            // 動態代理方法
            return Proxy.newProxyInstance(
                type.getClassLoader(),
                interfaces,
                new Plugin(target, interceptor, signatureMap));
        }
        return target;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
            Set<Method> methods = signatureMap.get(method.getDeclaringClass());
            if (methods != null && methods.contains(method)) {
                return interceptor.intercept(new Invocation(target, method, args));
            }
            return method.invoke(target, args);
        } catch (Exception e) {
            throw ExceptionUtil.unwrapThrowable(e);
        }
    }

    private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
        Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
        // issue #251
        if (interceptsAnnotation == null) {
            throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
        }
        Signature[] sigs = interceptsAnnotation.value();
        Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();
        for (Signature sig : sigs) {
            Set<Method> methods = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());
            try {
                Method method = sig.type().getMethod(sig.method(), sig.args());
                methods.add(method);
            } catch (NoSuchMethodException e) {
                throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
            }
        }
        return signatureMap;
    }

    private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
        Set<Class<?>> interfaces = new HashSet<>();
        while (type != null) {
            for (Class<?> c : type.getInterfaces()) {
                if (signatureMap.containsKey(c)) {
                    interfaces.add(c);
                }
            }
            type = type.getSuperclass();
        }
        return interfaces.toArray(new Class<?>[interfaces.size()]);
    }

}

讀取配置

private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
        for (XNode child : parent.getChildren()) {
            String interceptor = child.getStringAttribute("interceptor");
            Properties properties = child.getChildrenAsProperties();
            Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor().newInstance();
            interceptorInstance.setProperties(properties);
            configuration.addInterceptor(interceptorInstance);
        }
    }
}

public void addInterceptor(Interceptor interceptor) {
    interceptorChain.addInterceptor(interceptor);
}

僅僅是在 InterceptorChain 的 list 中增加一個元素的操作

工作原理

何時開始工作的

InterceptorChain 責任鏈調用應當是工作的開始，那麼查看 InterceptorChain.pluginAll() 方法應該是入口，找到調用地點就知道是何時開始工作的了

代碼是 Configuration 中提取的

public class Configuration {

	...
    public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
        ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
        parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
        return parameterHandler;
    }

    public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
                                                ResultHandler resultHandler, BoundSql boundSql) {
        ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
        resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
        return resultSetHandler;
    }

    public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
        StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
        statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
        return statementHandler;
    }

    public Executor newExecutor(Transaction transaction) {
        return newExecutor(transaction, defaultExecutorType);
    }

    public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
        executorType = executorType == null ? defaultExecutorType : executorType;
        executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
        Executor executor;
        if (ExecutorType.BATCH == executorType) {
            executor = new BatchExecutor(this, transaction);
        } else if (ExecutorType.REUSE == executorType) {
            executor = new ReuseExecutor(this, transaction);
        } else {
            executor = new SimpleExecutor(this, transaction);
        }
        if (cacheEnabled) {
            executor = new CachingExecutor(executor);
        }
        executor = (Executor) interceptorChain.pluginAll(executor);
        return executor;
    }
    ...
}

可見在新建 Executor、ParameterHandler、ResultSetHandler 以及 StatementHandler 時，都是返回的被代理過的對象

代理對象是什麼時候新建的呢？

• Executor
  • SqlSessionFactory 在 openSession 時調用，生命週期和 SqlSession 一致
• ParameterHandler
  • 在準備 Statement 的時候創建的，生命週期是一次SQL執行
• ResultSetHandler
  • 在準備 Statement 的時候創建的，是在 ParameterHandler 新建後立馬調用的，生命週期是一次SQL執行
• StatementHandler
  • 在 doQuery 的時候調用，生命週期是一次SQL執行

自定義

控制檯輸出 SQL 語句代替 log 日誌，並且記錄 SQL 執行時間

@Intercepts({ @Signature(type = StatementHandler.class, method = "query", args = { Statement.class, ResultHandler.class}) })
public class SQLInterceptor implements Interceptor {
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        long startTime = System.currentTimeMillis();
        StatementHandler statementHandler = (StatementHandler) invocation.getTarget();
        BoundSql boundSql = statementHandler.getBoundSql();
        String sql = boundSql.getSql();
        System.out.println("獲取到 SQL 語句："+sql);
        try {
            return invocation.proceed();
        }finally {
            System.out.println("SQL 執行耗時：" + (System.currentTimeMillis() - startTime) +"ms ");
        }
    }
}