目錄

Interceptor :
Intercepts:
Signature:
InterceptorChain:
Invocation:
Plugin:
PluginException:

動態(tài)代理

一定要動態(tài)代理相關(guān)的知識搞清楚.動態(tài)代理搞清楚了,看mybatis的plugin源碼,就會如魚得水.

什么是插件?

其實插件更準確的叫法應(yīng)該叫擴展點吧.哪些類和哪些方法可以被擴展,都是事先定義好的.用戶如何需要擴展,那么就按照規(guī)范,實現(xiàn)擴展的方法.
像下面的這個簡單的demo,我們支持Map的get是根據(jù)一個key,get相關(guān)的value,如果你要寫一個變態(tài)的方法,不管你get什么都返回"Always",那么你只要實現(xiàn)一個intercept,我們看到Invocation這個類,這個類是源碼里面被攔截的方法的調(diào)用參數(shù)相關(guān)信息,像返回"Always"這個攔截就沒有使用invocation,正常的攔截方法是,我們在invocation.proceed前搞點事情,或者要統(tǒng)計耗時,在proceed前后一起搞點事情.

我們先看看最簡單的用法.
我們要攔截的對象是Map的get方法,參數(shù)隨意.當我們寫單元測試get的時候,不管get什么都會返回Always.
因為我們的動態(tài)代理已經(jīng)重寫Map的get方法.

  @Intercepts({
      @Signature(type = Map.class, method = "get", args = {Object.class})})
  public static class AlwaysMapPlugin implements Interceptor {
    @Override
    public Object intercept(Invocation invocation) {
      //只是這個類沒有使用invocation,
      return "Always";
    }
  }
  @Test
  void mapPluginShouldInterceptGet() {
    Map map = new HashMap();
    map = (Map) new AlwaysMapPlugin().plugin(map);
    assertEquals("Always", map.get("Anything"));
  }

那么我們再來看看pagehelp是怎么使用的?

@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 {
}

攔截了哪些方法是不是一目了然.
然后我們再一步一步的往下跟蹤吧.

image.png

這里面會拼接limit參數(shù).

 ExecutorUtil.pageQuery(dialect, executor,
                        ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);

    public static <E> List<E> pageQuery(Dialect dialect, Executor executor, MappedStatement ms, Object parameter,
                                        RowBounds rowBounds, ResultHandler resultHandler,
                                        BoundSql boundSql, CacheKey cacheKey) throws SQLException {
        //判斷是否需要進行分頁查詢
        if (dialect.beforePage(ms, parameter, rowBounds)) {
            //生成分頁的緩存 key
            CacheKey pageKey = cacheKey;
            //處理參數(shù)對象
            parameter = dialect.processParameterObject(***) throws SQLException {
        //判斷是否需要進行分頁查詢
        if (dialect.beforePage(ms, parameter, rowBounds)) {
            //生成分頁的緩存 key
            CacheKey pageKey = cacheKey;
            //處理參數(shù)對象
            parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey);
            //調(diào)用方言獲取分頁 sql
            String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);
            BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);

            Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);
            //設(shè)置動態(tài)參數(shù)
            for (String key : additionalParameters.keySet()) {
                pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
            }
            //對 boundSql 的攔截處理
            if (dialect instanceof BoundSqlInterceptor.Chain) {
                pageBoundSql = ((BoundSqlInterceptor.Chain) dialect).doBoundSql(BoundSqlInterceptor.Type.PAGE_SQL, pageBoundSql, pageKey);
            }
            //執(zhí)行分頁查詢
            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
        } else {
            //不執(zhí)行分頁的情況下，也不執(zhí)行內(nèi)存分頁
            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
        }
    }
y);
            //調(diào)用方言獲取分頁 sql.這里已經(jīng)有l(wèi)imit了.
            String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);
            BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);

            Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);
            //設(shè)置動態(tài)參數(shù)
            for (String key : additionalParameters.keySet()) {
                pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
            }
            //對 boundSql 的攔截處理
            if (dialect instanceof BoundSqlInterceptor.Chain) {
                pageBoundSql = ((BoundSqlInterceptor.Chain) dialect).doBoundSql(BoundSqlInterceptor.Type.PAGE_SQL, pageBoundSql, pageKey);
            }
            //執(zhí)行分頁查詢.參數(shù)拼裝好了,交給executor去執(zhí)行了.
            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
        } else {
            //不執(zhí)行分頁的情況下，也不執(zhí)行內(nèi)存分頁
            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
        }
    }

MySqlDialect.getPageSql.

   @Override
    public String getPageSql(String sql, Page page, CacheKey pageKey) {
        StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);
        sqlBuilder.append(sql);
        if (page.getStartRow() == 0) {
            sqlBuilder.append("\n LIMIT ? ");
        } else {
            sqlBuilder.append("\n LIMIT ?, ? ");
        }
        return sqlBuilder.toString();
    }

如果我們要自己寫個分庫分表的組件,是不是也很容易實現(xiàn)了呢?

相關(guān)類分析

Interceptor

Object intercept(Invocation invocation) throws Throwable;
//target:需要攔截的方法,給target生成動態(tài)代理
 default Object plugin(Object target) {
    return Plugin.wrap(target, this);
  }

Plugin
其實我們不需要看這個類,只看第一個demo,就應(yīng)該知道Plugin肯定是實現(xiàn)了動態(tài)代理無非就是用的是jdk的還是cglib.

/**
 * 這個類是繼承了InvocationHandler的
 *
 * @author Clinton Begin
 */
public class Plugin implements InvocationHandler {

  //目標類
  private final Object target;
  //攔截器邏輯
  private final Interceptor interceptor;
  //方法簽名
  private final Map<Class<?>, Set<Method>> signatureMap;

  //私有的構(gòu)造器,不暴露出去.只有wrap才有權(quán)限調(diào)用.
  private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
    this.target = target;
    this.interceptor = interceptor;
    this.signatureMap = signatureMap;
  }

  //生成代理對象
  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) {
      //生成代理對象
      return Proxy.newProxyInstance(
        type.getClassLoader(),
        interfaces,
        new Plugin(target, interceptor, signatureMap));
    }
    //返回原對象,不生成代理
    return target;
  }

  //在 invoke() 方法中，Plugin 會檢查當前要執(zhí)行的方法是否在 signatureMap 集合中，
  //如果在其中的話，表示當前待執(zhí)行的方法是我們要攔截的目標方法之一，也就會調(diào)用 intercept() 方法執(zhí)行代理邏輯；
  //如果未在其中的話，則表示當前方法不應(yīng)被代理，直接執(zhí)行當前的方法即可。
  @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)) {
        //最終會執(zhí)行用戶傳入的攔截方法
        return interceptor.intercept(new Invocation(target, method, args));
      }
      //直接調(diào)用.
      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()]);
  }
}

InterceptorChain
看名字就知道用了責(zé)任鏈這個設(shè)計模式.實現(xiàn)就很簡單了,一個list攔截器集合,然后遍歷這個集合,一個一個的給這個target生成動態(tài)代理.
然后看到獲取攔截器的時候,返回的是一個不可變list,代碼就更安全了,不會被外部隨意的添加危險代碼.

public class InterceptorChain {

  //攔截器集合
  private final List<Interceptor> interceptors = new ArrayList<>();

  //核心:
  public Object pluginAll(Object target) {
    //
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

  //添加攔截器
  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }

  //獲取攔截器.
  public List<Interceptor> getInterceptors() {
    return Collections.unmodifiableList(interceptors);
  }
}

mybatis里plugin再哪里裝配的?

我們看一下這個調(diào)用方法,有四個地方,剛好是mybatis的四大對象.

image.png

  //下面幾個方法是四大對象的創(chuàng)建過程:我們可以發(fā)現(xiàn)，這幾個類對象的創(chuàng)建過程都調(diào)用了 InteceptorChain 的 pluginAll() 方法。
  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    //生成對象
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    //加入插件
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }

plugin包里就這幾個類,但是實現(xiàn)了及其靈活的擴展性,這個包雖然類很少,但是類的設(shè)計和類里面的代碼都及其的優(yōu)雅,各位讀者們,趕快去閱讀下大牛的代碼吧.