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This article introduces the relevant knowledge of "what are the interview questions about Java IO". In the operation of actual cases, many people will encounter such a dilemma. Next, let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Common classes in 1.IO, such as byte stream, character stream, interface, implementation class, method blocking?

The input stream is input from external files to memory, and the output stream is mainly output from memory to files.

For the common classes in IO, the first impression is that there are many classes in the IO stream, and the IO stream is mainly divided into character stream and byte stream. There are abstract classes InputStream and OutputStream in the character stream, their subclasses FileInputStream,FileOutputStream,BufferedOutputStream, and so on. Character stream BufferedReader and Writer and so on. All implement Closeable, Flushable, Appendable interfaces. The input and output in the program are saved in the form of a stream, and what is saved in the stream is actually all byte files.

The blocking method in java means that when the program calls the modified method, it must wait for the input data to be available or detect the end of the input or throw an exception, otherwise the program will stay on the statement and will not execute the following statement. Such as the read () and readLine () methods.

two。 What do you know about NIO?

For NIO, it is non-blocking, the core class:

1.Buffer provides (Buffer) caching support for all primitive types.

2.Charset character set Encoding and Decoding solution

A new original 3.Channel O abstraction for reading and writing Buffer types. A channel can be thought of as a connection to a specific device, program, or network.

3. What is the difference between byte stream and character stream?

The use of character stream and byte stream is very similar, but in fact, the operation of byte stream does not go through the buffer (memory) but directly manipulates the text itself, while the operation of character stream first passes through the buffer (memory) and then manipulates the file through the buffer.

Input and output data in bytes, and byte stream transmitted according to 8 bits

Input and output data in units of characters, and character streams are transmitted according to 16 bits

What is the difference between 4.NIO and traditional IO?

1, the traditional IO is generally a thread waiting for a connection, and after the connection comes, it is assigned to the processor thread. After the processor thread connects with the channel, if the channel has no data, it will block (the thread passively hangs) and cannot do anything else. NIO is different. First, uninterested events are filtered out in the process of polling by processor threads. Second, read and write that handle interesting events in Selector are returned directly by non-blocking operations, and the thread is not suspended.

2. The traditional io pipeline is unidirectional, while the nio pipeline is bi-directional.

3. Both are synchronous, that is, java programs read and write data personally. No matter traditional io or nio requires read and write methods, these are called by java programs rather than by the system. This has been improved in nio2.0, even with four asynchronous non-blocking AsynchronousXXX classes.

What are the differences between 5.BIO and NIO and AIO and their application scenarios?

Synchronization: java handles the io himself.

Async: java hands the io to the operating system to process, tells the cache size, and the processing completes the callback.

Blocking: when blocking IO is used, the Java call blocks until the read and write is complete.

Non-blocking: when using non-blocking IO, if you cannot read or write immediately, the Java call will return immediately. When the IO event dispatcher notifies you that you can read and write, you will read and write and cycle until the read and write is complete.

BIO: synchronize and block, the implementation mode of the server is to connect one thread at a time. An obvious defect of this mode is that because the number of client connections is proportional to the number of server threads, it may cause unnecessary thread overhead and, in serious cases, lead to server memory overflow. Of course, this situation can be improved through the thread pool mechanism, but it does not essentially eliminate this drawback.

NIO: before JDK1.4, Java's IO model was always BIO, but starting with JDK1.4, JDK introduced a new IO model, NIO, which is synchronously non-blocking. The implementation mode of the server is multiple requests for one thread, that is, the request will be registered with the multiplexer Selector, and the multiplexer will start a thread processing only when the multiplexer polls that the connection has an IO request.

AIO:JDK1.7 released NIO2.0, which is the true sense of asynchronous non-blocking. The implementation mode of the server is multiple valid requests for a thread. All client IO requests are completed by OS and then notify the server application to start thread processing (callback).

Application scenario: BIO is used when the number of concurrent connections is small, because it is very simple to program and debug, but if high concurrency is involved, you should choose NIO or AIO. A better suggestion is to use a mature network communication framework, Netty.

6. What is Java serialization and how to implement Java serialization?

Serialization is a mechanism for handling the flow of objects, streaming the contents of objects. The streaming object can be read and written, and the streaming object can be transferred between the networks. Serialization is to solve the problems caused by the read and write operations of the object stream

Serialization implementation: implement the Serialize interface for the classes that need to be serialized. There are no methods to implement. This interface is just to mark that the object can be serialized, and then use an output stream (such as: FileOutputStream) to construct an ObjectOutputStream (object stream) object, and then use the write (Object obj) method of the ObjectOutputStream object to write out the object of the parameter obj.

Comparison of 7.PrintStream, BufferedWriter and PrintWriter?

The output function of the PrintStream class is very powerful, and usually if you need to output text content, you should wrap the output stream as PrintStream and output it. It also provides two other functions. Unlike other output streams, PrintStream never throws an IOException;. Instead, the exception only sets the internal flag that can be tested by the checkError method. In addition, for automatic refresh, you can create a PrintStream

2. BufferedWriter: writes text to the character output stream, buffering individual characters to provide efficient writing of individual characters, arrays, and strings. The acquired characters can be output through the write () method, and then the newline operation can be performed through newLine (). The character stream in BufferedWriter must be brushed out by calling the flush method. And BufferedWriter can only operate on character streams. If you want to operate on a byte stream, use BufferedInputStream

3. The println method of PrintWriter automatically adds line breaks and does not throw exceptions. If you are concerned about exceptions, you need to call the checkError method to see if any exceptions occur. The PrintWriter construction method can specify parameters to automatically refresh the cache (autoflush).

8. What is the node flow, what is the processing flow, what are the advantages of each, and what are the characteristics of the creation of the processing flow?

The node stream is directly connected to the data source for input or output

Processing flow: processing it on the basis of node flow and extending some functions

The constructor that processes the flow must pass in a subclass of the node flow

9. What is an IO stream?

It is a stream of data from source to destination. For example, file copy, input stream and output stream are all included. The input stream reads the data from the file and stores it in the process (process), and the output stream reads the data from the process and writes it to the target file.

10. What Filter streams are available?

There are mainly four available filter Stream in the java.io package. Two bytes of filter stream, two characters filter stream. They are FilterInputStream, FilterOutputStream, FilterReader and FilterWriter. These classes are abstract and cannot be instantiated.

How many types of streams are there in 11.Java?

According to the direction of the stream: input stream (inputStream) and output stream (outputStream)

According to the implementation function: node flow (you can read and write data from or to a specific place (node). Such as FileReader) and processing flow (is the connection and encapsulation of an existing stream, through the function calls of the encapsulated stream to achieve data reading and writing. Such as BufferedReader. The constructor of the processing flow always takes another stream object as a parameter. A stream object is wrapped multiple times by other streams and is called a link to the stream.)

According to the units in which data is processed: byte stream and character stream. Byte streams inherit from InputStream and OutputStream, and character streams inherit from InputStreamReader and OutputStreamWriter.

twelve。 How to achieve object cloning?

There are two ways:

1)。 Implement the Cloneable interface and override the clone () method in the Object class

2)。 Implement the Serializable interface, achieve cloning through object serialization and deserialization, and achieve real deep cloning.

13. What is a buffer zone? What's the use?

A buffer is a special memory area, and in many cases, when a program needs to frequently manipulate a resource (such as a file or database), the performance will be very low, so in order to improve performance, you can temporarily read and write part of the data to the cache. Later, you can read and write data directly from this area, which can significantly improve performance.

Operations on Java character streams operate on buffers, so if we want to actively flush buffers to files in character stream operations, we can use the flush () method.

14. What is blocking IO? What is a non-blocking IO?

IO operations include: reading and writing to the hard disk, reading and writing to socket, and reading and writing to peripherals.

When the user thread initiates an IO request operation (this article takes the read request operation as an example), the kernel will check whether the data to be read is ready. For blocking IO, if the data is not ready, it will wait there until the data is ready; for non-blocking IO, if the data is not ready, it will return a flag message telling the user thread that the data to be read is not ready. When the data is ready, the data is copied to the user thread so that a complete IO read request operation is completed, that is, a complete IO read request operation consists of two phases:

1) check whether the data is ready

2) copy the data (the kernel copies the data to the user thread).

So the difference between blocking IO and non-blocking IO is that in the first phase, if the data is not ready, whether to wait or simply return a flag message while checking whether the data is ready.

The traditional IO in Java is blocking IO, such as reading data through socket. After calling the read () method, if the data is not ready, the current thread will block the read method call until the data is returned; in the case of non-blocking IO, when the data is not ready, the read () method should return a flag message telling the current thread that the data is not ready instead of waiting there.

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