Flink 源码之OperatorChain

Flink源码分析系列文档目录

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前言

OperatorChain是Flink中一个很重要的优化措施，能够将尽可能多的满足条件的数据处理操作在一个slot中串联完成，从而最小化线程执行上下文切换和网络通信，提高流计算系统性能。

Flink判断哪些操作可以纳入同一个chain的逻辑位于JobGraph生成过程中，详情请参见：Flink 源码之JobGraph生成。

名词解释

• StreamEdge：为StreamGraph拓扑图中的元素。StreamGraph由StreamNode和StreamEdge构成DAG图。详情参见 Flink 源码之StreamGraph生成。
• RecordWriterOutput：一种operator输出(Output)类型，用于将数据通过RecordWriter输出到ResultPartition。
• ChainingOutput：和RecordWriterOutput类似，也是一种operator输出类型，只不过ChainingOutput是在OperatorChain中专用的。它作为桥梁，将上游operator处理过得数据交给下游的operator。后面章节有详细分析。
• TypeSerializer：用于从DataInputView读取字节数组并反序列化为T类型，或者是将T类型序列化为DataOutputView。其中DataInputView和DataOutputView均直接操纵字节数组，这些字节数组的实际存储由MemorySegment支撑。
• StreamOperatorWrapper：用于包装StreamOperator，OperatorChain专用。它具有两个指针，分别指向前后两个operator，形成一个双向链表。Chain的概念由此而来。

接下来我们从OperatorChain的构造方法开始展开分析。

构造方法

OperatorChain在StreamTask中beforeInvoke方法构建出来（参见 Flink 源码之StreamTask）。获取chain到一起的operator（为OperatorChain中的mainOperator，如何生成chain到一起的operator的具体过程后面有分析），有数据到来的时候数据便交由mainOperator来处理。

OperatorChain的构造函数和分析如下所示：

public OperatorChain(
    StreamTask containingTask,
    RecordWriterDelegate>> recordWriterDelegate) {

    // 创建发送和接收OperatorEvent的Dispatcher
    this.operatorEventDispatcher =
        new OperatorEventDispatcherImpl(
        containingTask.getEnvironment().getUserCodeClassLoader().asClassLoader(),
        containingTask.getEnvironment().getOperatorCoordinatorEventGateway());

    // 获取用户代码类加载器
    final ClassLoader userCodeClassloader = containingTask.getUserCodeClassLoader();
    // 获取任务的配置
    final StreamConfig configuration = containingTask.getConfiguration();

    // 获取StreamTask的StreamOperator工厂
    StreamOperatorFactory operatorFactory =
        configuration.getStreamOperatorFactory(userCodeClassloader);

    // we read the chained configs, and the order of record writer registrations by output name
    // 获取OperatorChain中所有StreamOperator对应的StreamConfig，map的key为vertexID
    Map chainedConfigs =
        configuration.getTransitiveChainedTaskConfigsWithSelf(userCodeClassloader);

    // create the final output stream writers
    // we iterate through all the out edges from this job vertex and create a stream output
    // 按照数据流顺序，获取各个任务的StreamEdge
    List outEdgesInOrder = configuration.getOutEdgesInOrder(userCodeClassloader);
    Map> streamOutputMap =
        new HashMap<>(outEdgesInOrder.size());
    this.streamOutputs = new RecordWriterOutput[outEdgesInOrder.size()];

    // from here on, we need to make sure that the output writers are shut down again on failure
    boolean success = false;
    try {
        // 创建链式输出
        // 用于初始化streamOutputMap变量
        // streamOutputMap保存了每步操作的StreamEdge和output的对应关系
        createChainOutputs(
            outEdgesInOrder,
            recordWriterDelegate,
            chainedConfigs,
            containingTask,
            streamOutputMap);

        // we create the chain of operators and grab the collector that leads into the chain
        // 创建包含所有operatorWrapper的集合
        List> allOpWrappers =
            new ArrayList<>(chainedConfigs.size());
        // 创建mainOperator对应的output
        // OperatorChain的入口Operator为mainOperator
        // 这个operator通过ChainingOutput按照数据流向顺序串联了OperatorChain中的所有operator
        this.mainOperatorOutput =
            createOutputCollector(
            containingTask,
            configuration,
            chainedConfigs,
            userCodeClassloader,
            streamOutputMap,
            allOpWrappers,
            containingTask.getMailboxExecutorFactory());

        if (operatorFactory != null) {
            // 创建mainOperator和时间服务
            Tuple2> mainOperatorAndTimeService =
                StreamOperatorFactoryUtil.createOperator(
                operatorFactory,
                containingTask,
                configuration,
                mainOperatorOutput,
                operatorEventDispatcher);

            OP mainOperator = mainOperatorAndTimeService.f0;
            // 设置Watermark监控项
            mainOperator
                .getMetricGroup()
                .gauge(
                MetricNames.IO_CURRENT_OUTPUT_WATERMARK,
                mainOperatorOutput.getWatermarkGauge());
            // 创建mainOperatorWrapper
            this.mainOperatorWrapper =
                createOperatorWrapper(
                mainOperator,
                containingTask,
                configuration,
                mainOperatorAndTimeService.f1,
                true);

            // add main operator to end of chain
            // 将mainOperatorWrapper添加到chain的最后
            allOpWrappers.add(mainOperatorWrapper);

            // createOutputCollector方法将各个operator包装到operatorWrapper中
            // 按照数据流相反的顺序加入到allOpWrappers集合
            // 所以，尾部的operatorWrapper就是index为0的元素
            this.tailOperatorWrapper = allOpWrappers.get(0);
        } else {
            // 如果OperatorFactory为null
            checkState(allOpWrappers.size() == 0);
            this.mainOperatorWrapper = null;
            this.tailOperatorWrapper = null;
        }

        // 创建chain数据源
        this.chainedSources =
            createChainedSources(
            containingTask,
            configuration.getInputs(userCodeClassloader),
            chainedConfigs,
            userCodeClassloader,
            allOpWrappers);

        this.numOperators = allOpWrappers.size();

        // 将所有的StreamOperatorWrapper按照从上游到下游的顺序，形成双向链表
        firstOperatorWrapper = linkOperatorWrappers(allOpWrappers);

        success = true;
    } finally {
        // make sure we clean up after ourselves in case of a failure after acquiring
        // the first resources
        if (!success) {
            for (RecordWriterOutput output : this.streamOutputs) {
                if (output != null) {
                    output.close();
                }
            }
        }
    }
}

createChainOutputs

createChainOutputs方法作用为生成并保存每个StreamEdge和streamOutput的对应关系。代码如下所示：

private void createChainOutputs(
    List outEdgesInOrder,
    RecordWriterDelegate>> recordWriterDelegate,
    Map chainedConfigs,
    StreamTask containingTask,
    Map> streamOutputMap) {
    // 遍历已排序的StreamEdge
    for (int i = 0; i < outEdgesInOrder.size(); i++) {
        StreamEdge outEdge = outEdgesInOrder.get(i);

        // 创建streamOutput
        RecordWriterOutput streamOutput =
            createStreamOutput(
            recordWriterDelegate.getRecordWriter(i),
            outEdge,
            chainedConfigs.get(outEdge.getSourceId()),
            containingTask.getEnvironment());

        // 更新streamOutput数组
        this.streamOutputs[i] = streamOutput;
        // 保存每个StreamEdge和streamOutput的对应关系
        streamOutputMap.put(outEdge, streamOutput);
    }
}

接着继续分析createStreamOutput方法：

private RecordWriterOutput createStreamOutput(
    RecordWriter>> recordWriter,
    StreamEdge edge,
    StreamConfig upStreamConfig,
    Environment taskEnvironment) {
    // 获取Output标签，如果没有配置旁路输出，没有OutputTag
    OutputTag sideOutputTag = edge.getOutputTag(); // OutputTag, return null if not sideOutput

    TypeSerializer outSerializer = null;

    // 根据是否为旁路输出，获取对应的类型序列化器
    if (edge.getOutputTag() != null) {
        // side output
        outSerializer =
            upStreamConfig.getTypeSerializerSideOut(
            edge.getOutputTag(),
            taskEnvironment.getUserCodeClassLoader().asClassLoader());
    } else {
        // main output
        outSerializer =
            upStreamConfig.getTypeSerializerOut(
            taskEnvironment.getUserCodeClassLoader().asClassLoader());
    }

    // 返回创建的RecordWriterOutput
    return new RecordWriterOutput<>(recordWriter, outSerializer, sideOutputTag, this);
}

createOutputCollector

这个方法是chain的主要逻辑所在。我们重点分析。createOutputCollector方法分析如下：

private  WatermarkGaugeExposingOutput> createOutputCollector(
    StreamTask containingTask,
    StreamConfig operatorConfig,
    Map chainedConfigs,
    ClassLoader userCodeClassloader,
    Map> streamOutputs,
    List> allOperatorWrappers,
    MailboxExecutorFactory mailboxExecutorFactory) {
    List>, StreamEdge>> allOutputs =
        new ArrayList<>(4);

    // create collectors for the network outputs
    // 遍历非链式StreamEdge，非链式的StreamEdge输出需要走网络连接
    // 因此生成的Output类型为RecordWriterOutput
    for (StreamEdge outputEdge : operatorConfig.getNonChainedOutputs(userCodeClassloader)) {
        @SuppressWarnings("unchecked")
        // 从上一步createChainOutputs方法返回的streamOutputs中获取StreamEdge对应的output
        RecordWriterOutput output = (RecordWriterOutput) streamOutputs.get(outputEdge);
        // 加入到allOutputs集合中
        allOutputs.add(new Tuple2<>(output, outputEdge));
    }

    // Create collectors for the chained outputs
    // 获取该Operator对应的所有chained StreamEdge
    // 如果这个Operator具有多个chained的下游，这里会获取到多个outEdge
    for (StreamEdge outputEdge : operatorConfig.getChainedOutputs(userCodeClassloader)) {
        int outputId = outputEdge.getTargetId();
        // 获取这个outputEdge对应的StreamConfig
        StreamConfig chainedOpConfig = chainedConfigs.get(outputId);

        // 根据StreamEdge生成streamOutput，为WatermarkGaugeExposingOutput类型
        // WatermarkGaugeExposingOutput包装了Output和一个监控watermark的仪表盘
        // 如果存在可以chain的operator，需要递归调用，将下游与上游链接起来
        WatermarkGaugeExposingOutput> output =
            createOperatorChain(
            containingTask,
            chainedOpConfig,
            chainedConfigs,
            userCodeClassloader,
            streamOutputs,
            allOperatorWrappers,
            outputEdge.getOutputTag(),
            mailboxExecutorFactory);
        // 将其加入allOutputs集合中
        allOutputs.add(new Tuple2<>(output, outputEdge));
    }

    // 如果输出只有一个，返回这个输出
    if (allOutputs.size() == 1) {
        return allOutputs.get(0).f0;
    } else {
        // send to N outputs. Note that this includes the special case
        // of sending to zero outputs
        // 如果有多个输出，将allOutputs转换为Output类型数组
        @SuppressWarnings({"unchecked"})
        Output>[] asArray = new Output[allOutputs.size()];
        for (int i = 0; i < allOutputs.size(); i++) {
            asArray[i] = allOutputs.get(i).f0;
        }

        // This is the inverse of creating the normal ChainingOutput.
        // If the chaining output does not copy we need to copy in the broadcast output,
        // otherwise multi-chaining would not work correctly.
        // 根据配置中对象是否可重用，创建不同的OutputCollector
        if (containingTask.getExecutionConfig().isObjectReuseEnabled()) {
            // 在StreamRecord发往下游的时候实际发送的是StreamRecord的浅拷贝
            // 避免使用深拷贝，从而提高性能，但是需要注意如果开启ObjectReuse
            // 避免在下游改变流数据元素的值，否则会出现线程安全问题
            return new CopyingBroadcastingOutputCollector<>(asArray, this);
        } else {
            return new BroadcastingOutputCollector<>(asArray, this);
        }
    }
}

然后需要分析createOperatorChain方法。它将OperatorChain中所有的Operator包装为StreamOperatorWrapper类型，按照数据流反方向存入allOperatorWrappers集合。根据operator的顺序，依次生成ChainingOutput，将各个operator数据流串联起来。该方法内容如下：

private  WatermarkGaugeExposingOutput> createOperatorChain(
    StreamTask containingTask,
    StreamConfig operatorConfig,
    Map chainedConfigs,
    ClassLoader userCodeClassloader,
    Map> streamOutputs,
    List> allOperatorWrappers,
    OutputTag outputTag,
    MailboxExecutorFactory mailboxExecutorFactory) {
    // create the output that the operator writes to first. this may recursively create more
    // operators
    // 这里的operatorConfig为前一个方法中每次遍历的chainedOpConfig
    // 这里存在一个递归调用，将下游outEdge对应的StreamConfig作为参数，再次调用createOutputCollector
    // 最终的效果为上游operator的output指向下游operator，实现了chain，即链式调用
    // 最先返回的是最下游的output
    // operator的output按照从下游到上游的顺序，依次被包装为WatermarkGaugeExposingOutput
    WatermarkGaugeExposingOutput> chainedOperatorOutput =
        createOutputCollector(
        containingTask,
        operatorConfig,
        chainedConfigs,
        userCodeClassloader,
        streamOutputs,
        allOperatorWrappers,
        mailboxExecutorFactory);

    // 创建链式operator
    // 参数中使用上一步生成的operator output
    OneInputStreamOperator chainedOperator =
        createOperator(
        containingTask,
        operatorConfig,
        userCodeClassloader,
        chainedOperatorOutput,
        allOperatorWrappers,
        false);

    // 将operator包装到output中并返回，后面分析
    return wrapOperatorIntoOutput(
        chainedOperator, containingTask, operatorConfig, userCodeClassloader, outputTag);
}

createOperator方法根据operatorConfig创建出StreamOperator，然后使用StreamOperatorWrapper包装：

private > OP createOperator(
    StreamTask containingTask,
    StreamConfig operatorConfig,
    ClassLoader userCodeClassloader,
    WatermarkGaugeExposingOutput> output,
    List> allOperatorWrappers,
    boolean isHead) {

    // now create the operator and give it the output collector to write its output to
    // 使用StreamOperatorFactory创建出一个StreamOperator，使用指定的output
    // 这个方法比较复杂，这里不再介绍
    Tuple2> chainedOperatorAndTimeService =
        StreamOperatorFactoryUtil.createOperator(
        operatorConfig.getStreamOperatorFactory(userCodeClassloader),
        containingTask,
        operatorConfig,
        output,
        operatorEventDispatcher);

    // 获取创建的operator
    OP chainedOperator = chainedOperatorAndTimeService.f0;
    // 使用StreamOperatorWrapper包装此新创建的operator
    // StreamOperatorWrapper是operator在chaining时执行专用的封装类型，后面分析
    // 由于是递归调用，最先执行到这里的是最下游的算子
    // 因此allOperatorWrappers保存的顺序实际上是operator按照数据流向反向排列
    allOperatorWrappers.add(
        createOperatorWrapper(
            chainedOperator,
            containingTask,
            operatorConfig,
            chainedOperatorAndTimeService.f1,
            isHead));

    // 添加一个watermark监控用仪表盘
    chainedOperator
        .getMetricGroup()
        .gauge(
        MetricNames.IO_CURRENT_OUTPUT_WATERMARK,
        output.getWatermarkGauge()::getValue);
    return chainedOperator;
}

这里我们重点说下createOperatorWrapper。该方法使用StreamOperatorWrapper将StreamOperator包装起来。大家可能会问为什么这里需要使用StreamOperatorWrapper。我们看下StreamOperatorWrapper中的部分属性和方法。

private StreamOperatorWrapper previous;

private StreamOperatorWrapper next;

// 中间省略...

public void close(StreamTaskActionExecutor actionExecutor, boolean isStoppingBySyncSavepoint)
    throws Exception {
    if (!isHead && !isStoppingBySyncSavepoint) {
        // NOTE: This only do for the case where the operator is one-input operator. At present,
        // any non-head operator on the operator chain is one-input operator.
        actionExecutor.runThrowing(() -> endOperatorInput(1));
    }

    quiesceTimeServiceAndCloseOperator(actionExecutor);

    // propagate the close operation to the next wrapper
    if (next != null) {
        next.close(actionExecutor, isStoppingBySyncSavepoint);
    }
}

我们不难发现StreamOperatorWrapper具有previous和next两个指针，形成了一个双向链表。OperatorChain中的所有的operator保存在这种双向链表结构中，从而实现了chain的语义，即OperatorChain中的operator按照顺序依次执行。

createOperatorWrapper方法仅仅是使用StreamOperatorWrapper包装了StreamOperator，并没有生成双向队列。构建双向队列的方法为linkOperatorWrappers，后面我们分析。

除此以外createOperatorWrapper还具有自己的关闭逻辑。如close方法所示，它除了关闭当前operator外，还会递归关闭队列后面所有的operator。

最后我们分析。wrapOperatorIntoOutput方法。它将operator包装到Output中。output的类型为ChainingOutput。

private  WatermarkGaugeExposingOutput> wrapOperatorIntoOutput(
    OneInputStreamOperator operator,
    StreamTask containingTask,
    StreamConfig operatorConfig,
    ClassLoader userCodeClassloader,
    OutputTag outputTag) {

    WatermarkGaugeExposingOutput> currentOperatorOutput;
    // 如果开启了对象重用，创建ChainingOutput
    // 具体ChainingOutput相关内容在接下来章节分析
    if (containingTask.getExecutionConfig().isObjectReuseEnabled()) {
        currentOperatorOutput = new ChainingOutput<>(operator, this, outputTag);
    } else {
        // 否则创建CopyingChainingOutput
        // 传递StreamRecord时会进行深拷贝
        TypeSerializer inSerializer =
            operatorConfig.getTypeSerializerIn1(userCodeClassloader);
        currentOperatorOutput =
            new CopyingChainingOutput<>(operator, inSerializer, outputTag, this);
    }

    // wrap watermark gauges since registered metrics must be unique
    // 创建一个watermark监控仪表
    operator.getMetricGroup()
        .gauge(
        MetricNames.IO_CURRENT_INPUT_WATERMARK,
        currentOperatorOutput.getWatermarkGauge()::getValue);

    return currentOperatorOutput;
}

ChainingOutput

ChainingOutput实现了把上游operator的输出作为下一个operator的输入。创建ChainingOutput时需要传入下游operator，保存到input属性中。它的构造方法如下所示：

protected final Input input;

public ChainingOutput(
    OneInputStreamOperator operator,
    StreamStatusProvider streamStatusProvider,
    @Nullable OutputTag outputTag) {
    this(
        operator,
        (OperatorMetricGroup) operator.getMetricGroup(),
        streamStatusProvider,
        outputTag,
        operator::close);
}

public ChainingOutput(
    Input input,
    OperatorMetricGroup operatorMetricGroup,
    StreamStatusProvider streamStatusProvider,
    @Nullable OutputTag outputTag,
    @Nullable AutoCloseable closeable) {
    this.input = input;
    this.closeable = closeable;

    {
        Counter tmpNumRecordsIn;
        try {
            OperatorIOMetricGroup ioMetricGroup = operatorMetricGroup.getIOMetricGroup();
            tmpNumRecordsIn = ioMetricGroup.getNumRecordsInCounter();
        } catch (Exception e) {
            LOG.warn("An exception occurred during the metrics setup.", e);
            tmpNumRecordsIn = new SimpleCounter();
        }
        numRecordsIn = tmpNumRecordsIn;
    }

    this.streamStatusProvider = streamStatusProvider;
    this.outputTag = outputTag;
}

为了证实下ChainingOutput的确会把数据输出到下游operator，我们查看collect方法：

@Override
public void collect(StreamRecord record) {
    if (this.outputTag != null) {
        // we are not responsible for emitting to the main output.
        return;
    }

    pushToOperator(record);
}

protected  void pushToOperator(StreamRecord record) {
    try {
        // we know that the given outputTag matches our OutputTag so the record
        // must be of the type that our operator expects.
        @SuppressWarnings("unchecked")
        StreamRecord castRecord = (StreamRecord) record;

        numRecordsIn.inc();
        input.setKeyContextElement(castRecord);
        input.processElement(castRecord);
    } catch (Exception e) {
        throw new ExceptionInChainedOperatorException(e);
    }
}

collect方法调用了pushToOperator方法。其中执行了input.processElement(castRecord)，从而把数据传递给了下一个operator。

ChainingOutput还有一个子类叫做CopyingChainingOutput。它重写了pushToOperator方法，在数据发送往下游operator之前会创建一个深拷贝。如果启用了Object重用（containingTask.getExecutionConfig().isObjectReuseEnabled()返回true），使用ChainingOutput，否则使用CopyingChainingOutput。

@Override
protected  void pushToOperator(StreamRecord record) {
    try {
        // we know that the given outputTag matches our OutputTag so the record
        // must be of the type that our operator (and Serializer) expects.
        @SuppressWarnings("unchecked")
        StreamRecord castRecord = (StreamRecord) record;

        numRecordsIn.inc();
        // 这里创建出一个深拷贝，再发往下游
        StreamRecord copy = castRecord.copy(serializer.copy(castRecord.getValue()));
        input.setKeyContextElement(copy);
        input.processElement(copy);
    } catch (ClassCastException e) {
        // ...
    } catch (Exception e) {
        throw new ExceptionInChainedOperatorException(e);
    }
}

createChainedSources

该方法用于创建chained数据源。

@SuppressWarnings("rawtypes")
private Map createChainedSources(
    StreamTask containingTask,
    InputConfig[] configuredInputs,
    Map chainedConfigs,
    ClassLoader userCodeClassloader,
    List> allOpWrappers) {
    // 如果所有的configuredInputs都不是SourceInputConfig类型，返回空map
    if (Arrays.stream(configuredInputs)
        .noneMatch(input -> input instanceof SourceInputConfig)) {
        return Collections.emptyMap();
    }
    // chained 数据源只适用于多个输入的StreamOperator
    checkState(
        mainOperatorWrapper.getStreamOperator() instanceof MultipleInputStreamOperator,
        "Creating chained input is only supported with MultipleInputStreamOperator and MultipleInputStreamTask");
    Map chainedSourceInputs = new HashMap<>();
    MultipleInputStreamOperator multipleInputOperator =
        (MultipleInputStreamOperator) mainOperatorWrapper.getStreamOperator();
    // 获取它所有的Input
    List operatorInputs = multipleInputOperator.getInputs();

    // 计算InputGate的Index，为所有InputGate的index最大值加1
    int sourceInputGateIndex =
        Arrays.stream(containingTask.getEnvironment().getAllInputGates())
        .mapToInt(IndexedInputGate::getInputGateIndex)
        .max()
        .orElse(-1)
        + 1;

    // 遍历每个Input
    for (int inputId = 0; inputId < configuredInputs.length; inputId++) {
        // 排除掉所有不是SourceInputConfig类型的情况
        if (!(configuredInputs[inputId] instanceof SourceInputConfig)) {
            continue;
        }
        SourceInputConfig sourceInput = (SourceInputConfig) configuredInputs[inputId];
        int sourceEdgeId = sourceInput.getInputEdge().getSourceId();
        // 根据input edge获取sourceInputConfig
        StreamConfig sourceInputConfig = chainedConfigs.get(sourceEdgeId);
        OutputTag outputTag = sourceInput.getInputEdge().getOutputTag();

        // 创建链式的数据源output
        // 目前只支持Object Reuse开启
        // 实际返回的类型为ChainingOutput
        WatermarkGaugeExposingOutput chainedSourceOutput =
            createChainedSourceOutput(
            containingTask,
            operatorInputs.get(inputId),
            (OperatorMetricGroup) multipleInputOperator.getMetricGroup(),
            outputTag);

        // 创建数据源operator
        // createOperator前面分析过，不再赘述
        SourceOperator sourceOperator =
            (SourceOperator)
            createOperator(
            containingTask,
            sourceInputConfig,
            userCodeClassloader,
            (WatermarkGaugeExposingOutput>)
            chainedSourceOutput,
            allOpWrappers,
            true);
        // 放入chainedSourceInputs中
        chainedSourceInputs.put(
            sourceInput,
            new ChainedSource(
                chainedSourceOutput,
                new StreamTaskSourceInput<>(
                    sourceOperator, sourceInputGateIndex++, inputId)));
    }
    return chainedSourceInputs;
}

linkOperatorWrappers

linkOperatorWrappers方法将chain中的operator按照逐个连接起来。注意，由于上一步createOutputCollector方法构造的allOperatorWrappers存放的各个operator顺序为从下游到上游，因此linkOperatorWrappers方法需要将这个连接顺序颠倒过来。

private StreamOperatorWrapper linkOperatorWrappers(
    List> allOperatorWrappers) {
    // 暂存前一个处理的operator
    StreamOperatorWrapper previous = null;
    // 遍历所有的operator
    for (StreamOperatorWrapper current : allOperatorWrappers) {
        if (previous != null) {
            // 设置previous的前一个operator为当前operator
            previous.setPrevious(current);
        }
        // 设置当前operator的下一个operator为previous
        current.setNext(previous);
        // 设置当前operator为previous
        previous = current;
    }
    return previous;
}

initializeStateAndOpenOperators

在StreamTask开始接收数据之前，需要初始化各个operator的状态（state）和开启operator（调用各个operator的open方法）。initializeStateAndOpenOperators正是用来完成这个工作的。

protected void initializeStateAndOpenOperators(
    StreamTaskStateInitializer streamTaskStateInitializer) throws Exception {
    for (StreamOperatorWrapper operatorWrapper : getAllOperators(true)) {
        StreamOperator operator = operatorWrapper.getStreamOperator();
        operator.initializeState(streamTaskStateInitializer);
        operator.open();
    }
}

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