How to use Java to capture data from the network

With the advent of the Internet era, the generation and sharing of large amounts of data has become a trend. In order to make better use of this data, learning how to crawl data from the Internet has become one of the necessary skills. This article will introduce how to use Java to implement network crawling data.

1. Basic knowledge of web crawling data

Web crawling data simply means accessing some designated websites through the network, and then obtaining the required data from the website and performing storage. This process is actually a process in which the client sends a request to the server, and the server responds to the request and returns data.

When the client sends a request to the server, you need to pay attention to the following:

1. Format of data: The request needs to know the type of data returned by the server, such as: HTML, JSON, etc.
2. Request header information: In order to indicate the identity of the client and the specific information of the request, the request header information needs to be passed to the server.
3. Request parameters: Some websites will require the client to provide some parameters to return data correctly, such as search keywords, etc.
4. Response status code: The response status code returned by the server to the client can help us confirm the success or failure of the request.

2. Steps to use Java to capture data from the network

1. Establish a connection

To use Java to capture data from the network, we first need to establish the target Website links. Java provides a URL class. By instantiating this class, we can get an object representing the connection. For example:

URL url = new URL("https://www.example.com");

2. Open the connection

After establishing the connection, we need to open This connection is prepared to send a request to get the data returned from the server. In Java, you can open a connection and return a URLConnection object through the URL object openConnection() method, for example:

URLConnection connection = url.openConnection();

3. Set request header information

Before sending the request, we need to provide the request header information to the server. In Java, it can be set through the setRequestProperty() method of the URLConnection class:

connection.setRequestProperty("User-Agent","Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML , like Gecko) Chrome/83.0.4103.61 Safari/537.36");

The first parameter is the name of the header information, and the second parameter is the value of the header information.

4. Send a request

After setting the request header information, we can call the connect() method of the URLConnection class to establish a connection with the target server. For example:

connection.connect();

5. Get response information

After the server responds, we need to obtain and process the data returned from the server. URLConnection provides a getInputStream() method to return an input stream object from which the returned data can be read. For example:

InputStream inputStream = connection.getInputStream();

6. Responsibility chain mode encapsulation

In order to improve the efficiency of data capture and make the code structure clearer, You can consider using the chain of responsibility pattern to encapsulate the entire process of capturing data. For example:

public class DataLoader {

private Chain chain;

public DataLoader() {
    chain = new ConnectionWrapper(new HeaderWrapper(new RequestWrapper(new ResponseWrapper(null))));
}

public String load(String url) {
    return chain.process(url);
}

}

Among them, the ConnectionWrapper, HeaderWrapper, RequestWrapper and ResponseWrapper classes represent the four links of connection, request header, request and response respectively. , they all implement the same Chain interface, and in the constructor, they are passed from one to the next, ultimately forming a chain of responsibility. The load() method accepts a url string as a parameter and finally returns a string type result. When loading, you only need to call the load() method of the instance of the DataLoader class.

3. Precautions

1. Pay attention to the anti-crawler mechanism of the website and do not grab a large amount of data at once, otherwise the IP address may be banned.
2. Pay attention to the website's data request method. Some websites may require a specific request method to return data correctly.
3. When processing the returned data, it needs to be parsed accordingly according to the returned data format. There are also differences in the parsing methods of different formats. For example, XML needs to be parsed using DOM or SAX, and JSON needs to be parsed using libraries such as GSON or Jackson.

4. Summary

This article introduces how to use Java to capture data from the network. It should be noted that web scraping is a resource-intensive operation. If a large amount of data is accidentally scraped, it may put pressure on the server. Therefore, web scraping needs to be done in compliance with internet ethics and under appropriate circumstances.

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