Shulou(Shulou.com)06/01 Report--

Today, I would like to talk to you about the understanding of BIO,NIO,AIO in JAVA. Many people may not understand it very well. In order to make you understand better, the editor has summarized the following contents for you. I hope you can get something from this article.

In the design of high-performance IO systems, there are several noun concepts that often confuse us. The details are as follows:

Numbered noun explanation example 1 synchronization means that the user process triggers the IO operation and waits or polls to see if the IO operation is ready to go shopping for clothes, do it yourself, and can't do anything else. 2 Asynchronous Asynchronous means that the user process starts to do its own things after triggering the IO operation, and when the IO operation has been completed, it will be notified of the completion of the IO (the characteristic of async is the notification) to tell friends that they are the right size, size and color of the clothes, and then they can do something else. (when using asynchronous IO, Java delegates IO read and write to OS, and needs to pass the data buffer address and size to OS.) 3 blocking means that when trying to read and write to the file descriptor, if there is nothing to read or write at that time, the program goes into a waiting state until there is something to read or write to go to the bus station to recharge. The recharger is not here (he may have gone to the toilet), and then we wait here until the recharger comes back. Of course, this is not the case in the real world, but it is true in computers. (4) in the non-blocking and non-blocking state, if there is nothing to read or cannot write, the read-write function will return immediately instead of waiting. When you withdraw money from the bank for business, you will get a small ticket. After receiving it, we can play with our mobile phone or chat with others. When it is our turn, the bank horn will notify us, and then we can go.

After checking some materials, I would like to try to explain these nouns in an easy-to-understand way. no, no, no.

Before figuring out the above questions, we first have to understand what is synchronous, asynchronous, blocking, and non-blocking. Only these individual concepts are understood clearly, and then in combination, it is relatively easy to understand.

1. Synchronization and asynchronism are for the interaction between the application and the kernel.

2. Blocking and non-blocking are different ways for the process to access data according to the ready state of the IO operation. To put it bluntly, it is a way to implement the read or write operation function. In the blocking mode, the read or write function will always wait, while in the non-blocking mode, the read or write function will immediately return a status value.

From the above description, we can basically sum up a short sentence, synchronous and asynchronous are the purpose, blocking and non-blocking are the ways of implementation.

Now let's understand the IO type of the combination mode, which is much easier to understand.

Synchronous blocking IO (JAVA BIO):

Synchronization and blocking, the server implementation mode is to connect one thread at a time, that is, when the client has a connection request, the server needs to start a thread for processing. If the connection does nothing, it will cause unnecessary thread overhead. Of course, it can be improved through the thread pool mechanism.

Synchronous non-blocking IO (Java NIO): synchronous non-blocking, the server implementation mode is one thread per request, that is, all connection requests sent by the client are registered with the multiplexer, and the multiplexer starts a thread for processing only when the multiplexer polls that the connection has an Icano request. The user process also needs to ask whether the IO operation is ready from time to time, which requires the user process to keep asking.

Asynchronous blocking IO (Java NIO):

In this way, it means that after the application initiates an IO operation, it does not wait for the completion of the kernel IO operation, and the kernel will notify the application after the IO operation is completed. In fact, this is the most critical difference between synchronization and asynchronism. Synchronization must wait or actively ask whether the IO is completed, so why is it blocked? Because this is done through select system calls, and the implementation of the select function itself is blocked, one advantage of using the select function is that it can listen on multiple file handles at the same time (if from a UNP point of view, select is a synchronous operation. Because after select, the process also needs to read and write data), thus improving the concurrency of the system!

(Java AIO (NIO.2)) Asynchronous non-blocking IO:

In this mode, the user process only needs to initiate an IO operation and return immediately. After the IO operation is really completed, the application will be notified of the completion of the IO operation. At this time, the user process only needs to process the data, and there is no need for the actual IO read and write operation, because the real IO read or write operation has been completed by the kernel.

Analysis of applicable scenarios for BIO, NIO and AIO:

The BIO method is suitable for the architecture with a small number of connections and a fixed number of connections, which requires high server resources, and concurrency is limited to the application. JDK1.4 is the only choice before, but the program is intuitive, simple and easy to understand.

NIO is suitable for architectures with a large number of connections and short connections (light operation), such as chat servers, where concurrency is limited to applications, programming is more complex, and JDK1.4 begins to support it.

AIO is used in architectures with a large number of connections and long connections (heavy operations), such as photo album server, fully calling OS to participate in concurrent operations, programming is more complex, and JDK7 begins to support it.

After figuring out the above concepts, let's go back to the Reactor pattern and the Proactor pattern.

In fact, both blocking and non-blocking can be understood as concepts that exist only in the context of synchronization. For asynchronism, blocking and non-blocking will no longer be divided. For the user process, after receiving the asynchronous notification, you can directly manipulate the data in the user state space of the process. )

First, let's take a look at the Reactor mode, which is applied to the scenario of synchronizing Imax O. Let's take a read operation and a write operation as examples to look at the specific steps in Reactor:

Read operation:

1. The application registers read-ready events and associated event handlers

two。 The event splitter waits for the event to occur

3. When a read-ready event occurs, the event splitter calls the event handler registered in the first step

4. The event handler first performs the actual read operation, and then performs further processing based on what is read

A write operation is similar to a read operation, except that the first step registers the write-ready event.

Let's take a look at the read and write operations in Proactor mode:

Read operation:

1. The application initializes an asynchronous read operation and then registers the corresponding event handler, which focuses not on the read ready event but on the read completion event, which is the key difference from Reactor.

two。 Event splitter waits for read operation to complete event

3. When the event splitter waits for the read operation to complete, the operating system calls the kernel thread to complete the read operation (asynchronous IO is the operating system responsible for reading and writing data to the buffer passed by the application for application operation, and the operating system plays an important role), and puts the read content into the cache passed by the user. This is also different from Reactor, where applications need to pass caches in Proactor.

4. After the event splitter captures the read completed event, it activates the event handler registered by the application, and the event handler reads the data directly from the cache without the actual read operation.

Write and read operations in Proactor, except that the event of interest is the write completion event.

As can be seen from the above, the main difference between Reactor and Proactor mode is who does the real read and write operation. In Reactor, the application needs to read or write data by itself, while in Proactor mode, the application does not need to read or write from the cache, and the operating system will read or write the cache to the real IO device.

To sum up, synchronization and asynchronism are relative to the interaction between the application and the kernel, synchronization needs to be asked actively, while when asynchronous, the kernel notifies the application when IO events occur, and blocking and non-blocking are only the ways in which the system implements functions when calling system calls.

If you want to have a Kung Pao chicken meal:

Synchronous blocking: you go to the restaurant to order, then wait there, and yell, "are you ready?"

Synchronous non-blocking: after ordering in a restaurant, you go to walk the dog. But after walking for a while, he went back to the restaurant and shouted, "are you ready?"

Asynchronous blocking: when walking your dog, you get a call from the restaurant saying that the meal is ready and ask you to get it yourself.

Asynchronous non-blocking: the restaurant called and said, we know your location, we will send it to you later, just walk the dog at ease.

"An IO operation is actually divided into two steps: initiating an IO request and the actual IO operation.

The difference between synchronous IO and asynchronous IO lies in whether the second step is blocked. If the actual IO reads and writes block the request process, it is synchronous IO.

The difference between blocking IO and non-blocking IO lies in whether the originating IO request will be blocked in the first step, traditional blocking IO if blocking until completion, and non-blocking IO if not blocking.

Synchronization and asynchronism are for the interaction between the application and the kernel. Synchronization means that the user process triggers the IO operation and waits or polls to see if the IO operation is ready, while asynchronous means that the user process starts to do its own thing after triggering the IO operation, and will be notified of the completion of the IO operation when the IO operation has been completed. Blocking and non-blocking are different ways for the process to access data according to the ready state of the IO operation. To put it bluntly, it is a way to implement the read or write operation function. In the blocking mode, the read or write function will always wait, while in the non-blocking mode, the read or write function will immediately return a status value.

Therefore, IO operations can be divided into three categories: synchronous blocking (that is, early IO operations), synchronous non-blocking (NIO), and asynchronous (AIO).

Synchronous blocking:

In this way, the user process must wait for the completion of the IO operation after initiating an IO operation, and the user process can run only after the IO operation is actually completed. JAVA's traditional IO model belongs to this approach.

Synchronous non-blocking:

In this way, the user process can return to do other things after initiating an IO operation, but the user process needs to ask whether the IO operation is ready from time to time, which requires the user process to keep asking, thus introducing unnecessary waste of CPU resources. The current NIO of JAVA belongs to synchronous non-blocking IO.

Async:

In this way, it means that after the application initiates an IO operation, it does not wait for the completion of the kernel IO operation and notifies the application after the kernel completes the IO operation. "

After reading the above, do you have any further understanding of the understanding of BIO,NIO,AIO in JAVA? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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