Kafka源码分析之KafkaProducer发送数据send()方法

        KafkaProducer是Kafka中Producer的一种实现，其主要功能就是发送消息给Kafka中broker。其send()方法如下：

    /**
     * Asynchronously send a record to a topic. Equivalent to <code>send(record, null)</code>.
     * See {@link #send(ProducerRecord, Callback)} for details.
     */
    @Override
    public Future<RecordMetadata> send(ProducerRecord<K, V> record) {
        return send(record, null);
    }

        再看两个参数的send()方法，代码如下：

    /**
     * Asynchronously send a record to a topic and invoke the provided callback when the send has been acknowledged.
     * <p>
     * The send is asynchronous and this method will return immediately once the record has been stored in the buffer of
     * records waiting to be sent. This allows sending many records in parallel without blocking to wait for the
     * response after each one.
     * <p>
     * The result of the send is a {@link RecordMetadata} specifying the partition the record was sent to and the offset
     * it was assigned.
     * <p>
     * Since the send call is asynchronous it returns a {@link java.util.concurrent.Future Future} for the
     * {@link RecordMetadata} that will be assigned to this record. Invoking {@link java.util.concurrent.Future#get()
     * get()} on this future will block until the associated request completes and then return the metadata for the record
     * or throw any exception that occurred while sending the record.
     * <p>
     * If you want to simulate a simple blocking call you can call the <code>get()</code> method immediately:
     *
     * <pre>
     * {@code
     * byte[] key = "key".getBytes();
     * byte[] value = "value".getBytes();
     * Produc