Collection of classic Java interview questions (4)

1. Application scenarios of reflection mechanism

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1. Reverse code, such as decompilation

2. Framework combined with annotations such as Retrofit

3. Simple reflection mechanism Application frameworks such as EventBus 2.x

4. Dynamically generated class frameworks such as Gson

2. What is multi-threading?

Multi-threading refers to the technology that realizes the concurrent execution of multiple threads from software or hardware. In a program, these independently running program fragments are called "threads", and the concept of programming using them is called "multi-threading".

Computers with multi-threading capabilities can execute more than one thread at the same time due to hardware support, thus improving overall processing performance.

#The main thread is the first thread generated in the creation process, which is the thread corresponding to the main function.

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3. Tell me about the benefits of multi-threading?

1. Advantages of multi-threading

• The cost of creating a new thread is much smaller than creating a new process

• Compared with switching between processes, switching between threads requires the operating system to do very little work

• Threads occupy much fewer resources than processes

• Can make full use of the parallel number of multi-processors

• After waiting for the slow IO operation to end, the program can perform other computing tasks

• Computing (CPU)-intensive applications, in order to run on multi-processor systems, the calculation is broken down into multiple threads to implement

• IO-intensive applications , in order to improve performance, IO operations are overlapped, and threads can wait for different IO operations.

2. Disadvantages of multi-threading

• Performance loss (a computing-intensive thread is rarely blocked by external events and cannot communicate with other Threads share the same processor. When the number of computationally intensive threads is more than the available processors, there may be a large performance loss. The performance loss here refers to the additional synchronization and scheduling overhead. Two available Resources remain unchanged.)

• The robustness is reduced (there is a lack of protection between threads. In a multi-threaded program, due to the slight difference in time allocation or the sharing of some incorrect The possible impact of adverse effects on variables that should be shared is very large.)

• Lack of access control (because the process is the basic granularity of access control, calling certain OS in a thread Functions will affect the entire process.)

• Increased programming difficulty (writing and debugging a multi-threaded program is much more difficult than a single-threaded program.)

4. What is the difference between threads and processes?

1. Scheduling

The process is a basic unit for the operating system to allocate resources. Threads are the basic unit of CPU scheduling.

2. Concurrency

After the introduction of threads, not only processes can be executed concurrently, but multiple threads in a process can also be executed concurrently, even allowing one All processes in the process execute concurrently.

Similarly, threads in different processes can also be executed concurrently. This enables the OS to have better concurrency, improving resource utilization and system throughput.

3. Owning resources

A process can own resources and is the basic unit of system resource ownership. The thread itself does not own system resources, only some resources that can ensure independent operation. This resource is private to each thread.

For example, thread ID, a set of registers, stack, errno, signal mask word (there is only one pending signal in a process, but any thread can handle this signal), scheduling priority.

4. Independence

The independence of threads in the same process is much lower than that in different processes.

5. System overhead

The cost of thread switching is lower than the cost of process switching.

6.Support multi-processor system

For traditional processes , that is, a single-threaded process, no matter how many processors there are, the process can only run on the same processor, but for a multi-threaded process, multiple threads in a process can be assigned to multiple processors, so that Its concurrent execution speeds up the completion of the process.

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5. Application scenarios of processes and threads

• Need to frequently create and destroy priority threads.

• Prioritize threads that require large amounts of calculations.

• Threads are used for strong correlations, and processes are used for weak correlations.

• It may be extended to use processes for multi-machine distribution and threads for multi-core distribution.

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