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This article mainly introduces "how the processes and threads of Java come from". In the daily operation, I believe that many people have doubts about how the processes and threads of Java come from. The editor consulted all kinds of materials and sorted out simple and easy-to-use methods of operation. I hope it will be helpful to answer the doubts of "how do the processes and threads of Java come from?" Next, please follow the editor to study!

one。 Why do processes appear in the operating system?

When it comes to the origin of the process, we need to start with the history of the development of the operating system.

Maybe today, we can't imagine what the computer looked like many years ago. We can now use computers to do a lot of things: office work, entertainment and surfing the Internet, but when computers first appeared, it was to solve the problem of mathematical calculation. because a lot of calculations are done through manpower is very time-consuming and labor-intensive.

In the beginning, the computer can only accept some specific instructions, and the user enters an instruction and the computer does an operation. When the user is thinking or entering data, the computer is waiting. This is obviously inefficient and inefficient, because most of the time, the computer is waiting for user input.

So can you write down a series of instructions that need to be operated in advance, form a list, and then give it to the computer at one time, and the computer constantly reads the instructions to carry out the corresponding operations? In this way, the batch operating system was born. The user can write multiple programs that need to be executed on magnetic tape, then leave them to the computer to read and execute them one by one, and write the output to another tape.

Although the birth of the batch operating system has greatly improved the convenience of task processing, there is still a big problem:

If there are two tasks An and B, task A needs to read a large amount of data input in the middle of execution (IWeiO operation), and CPU can only quietly wait for task A to finish reading the data before continuing to execute, which is a waste of CPU resources. So people think, can you let task B execute when task A reads the data, let task B pause after task A has finished reading the data, and then let task A continue to execute?

But there is a problem, it turns out that every time a program runs in the computer, that is to say, there is only one program running data in memory. And if you want task A to perform the Igamot O operation, let task B to execute, it is necessary to load multiple programs in memory, then how to deal with it? How to identify the data used by multiple programs? And when a program is paused, how does it return to the state it executed before?

At this time, people invented the process, using the process to correspond to a program, each process corresponds to a certain memory address space, and can only use its own memory space, each process does not interfere with each other. And the process saves the running state of the program at every moment, which makes it possible for the process to switch. When a process is temporary, it saves the state of the current process (such as process identity, resources used by the process, etc.), and the next time it is switched back, it is restored according to the previously saved state, and then continues to execute.

This is concurrency, which allows the operating system to look macroscopically as if there are multiple tasks performing at the same time. In other words, processes make concurrency of the operating system possible.

Note that although concurrency has multiple tasks executing from a macro point of view, in fact, only one task is consuming CPU resources at any given moment (for single-core CPU, of course).

two。 Why is there a thread?

After the emergence of the process, the performance of the operating system has been greatly improved. Although the emergence of processes has solved the concurrency problem of the operating system, people are still not satisfied, and people gradually have requirements for real-time performance. Because a process can only do one thing in a period of time, if a process has multiple subtasks, it can only perform these subtasks one by one.

For example, for a monitoring system, it should not only display image data on the screen, but also communicate with the server to obtain image data, but also deal with people's interactive operations. If at some point the system is communicating with the server to obtain image data, and the user clicks a button on the monitoring system, the system will have to wait for the image data to be obtained before it can process the user's operation. if it takes 10 seconds to obtain image data, then the user can only wait all the time. Obviously, people cannot be satisfied with such a system.

So is it possible to perform these subtasks separately? That is, when the system acquires image data, if the user clicks a button, it will suspend the acquisition of image data and respond to the user's operation first (because the user's operation often takes a short time). And then continue to get the image data.

People invented a thread to let a thread execute a subtask, so that a process includes multiple threads, each thread is responsible for an independent subtask, so that when the user clicks the button, the thread that acquires the image data can be suspended, and the UI thread can respond to the user's operation, and then switch back after the response, so that the thread that gets the image can get the CPU resources. This makes users feel that the system is doing many things at the same time, which meets the real-time requirements of users.

In other words, processes make the concurrency of the operating system possible, while threads make the internal concurrency of the process possible.

However, it should be noted that although a process includes multiple threads, these threads share the resources and address space occupied by the process. The process is the basic unit of resource allocation in the operating system, and the thread is the basic unit of scheduling in the operating system.

three。 Multithreading concurrency

Because multiple threads share the resources and address space of their processes, there is a problem:

What if multiple threads want to access a resource at the same time?

This problem is the synchronization problem that should be emphasized in the post-preface article.

Then some friends may ask that multithreaded programming is often used now, so is the performance of multithreading due to single threading?

Not necessarily, it depends on the specific task and the configuration of the computer. For example:

For single-core CPU, if it is a CPU-intensive task, such as extracting files, the performance of multithreading is not as good as that of single-threading, because decompressing files always takes up CPU resources. If multithreading is used, the overhead caused by thread switching will degrade the performance.

But for tasks such as interaction types, it must be necessary to use multithreading,

For multi-core CPU, multithreading is definitely better than single threading for decompressing files, because multiple threads can make better use of the resources of each core.

Although multithreading can improve program performance, its programming is much more complex than that of single thread, and thread safety should be considered. Therefore, in the actual programming process, we should make a specific choice according to the actual situation.

At this point, the study on "how the processes and threads of Java come from" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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