如何从 Spring 引导中的资源中读取 JSON 文件

Question

Using Spring Boot 2.1.5 Release, have created the following sample Spring Boot Microservice:

Maven Project Structure:

MicroService
    │
    pom.xml
    src
    │
    └───main
        │ 
        ├───java
        │   │ 
        │   └───com
        │       └───microservice
        │           │
        │           └───MicroServiceApplication.java  
        │  
        └───resources
            │
            └───data.json
                │                    
                application.properties

Have the following JSON file (inside src/main/resources/data.json):

{"firstName": "John", "lastName": "Doe"}

MicroServiceApplication:

@SpringBootApplication
public class MicroServiceApplication {

    @Bean
    CommandLineRunner runner() {
        return args -> {
            String data = FilePathUtils.readFileToString("../src/main/resources/data.json", MicroServiceApplication.class);
            System.out.println(data);
        };
    }

    public static void main(String[] args) {
        SpringApplication.run(MicroServiceApplication.class, args);
    }
}

Throws the following exception:

  java.lang.IllegalStateException: Failed to execute CommandLineRunner
  ...
  Caused by: java.io.IOException: Stream is null

FilePathUtils.java:

import io.micrometer.core.instrument.util.IOUtils;

import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.Charset;

public class FilePathUtils {
    public static String readFileToString(String path, Class aClazz) throws IOException {

        try (InputStream stream = aClazz.getClassLoader().getResourceAsStream(path)) {
            if (stream == null) {
                throw new IOException("Stream is null");
            }
            return IOUtils.toString(stream, Charset.defaultCharset());
        }
    }
}

What am I possibly being doing wrong?

Answer 1

While @Deadpool has provided the answer, I would like to add that when the artifact of spring boot is created there is no such src/main/ folder anymore (you can open the spring boot artifact and make sure by yourself).

So you can't load resources like this:

FilePathUtils.readFileToString("../src/main/resources/data.json", MicroServiceApplication.class);

Spring indeed has an abstraction called Resource that can be used in the application, that can even be injected into the classes / configuration:

@Value("classpath:data/data.json")
Resource resourceFile;

Notice the prefix "classpath" it means that the resource will be resolved from the classpath (read everything properly packaged into the artifact).

There is a pretty good Tutorial that can be handy

Answer 2

In spring boot project you can use ResourceUtils

Path file = ResourceUtils.getFile("data/data.json").toPath();

or ClassPathResource

String clsPath =  new ClassPathResource("data/data.json").getPath();

Sometimes if you are reading different extension file like .graphql or .mmdb or .json you need to read it as InputStream using spring ResourceLoader , this article has clear explanation

@Autowire
private ResourceLoader resourceLoader;

   Resource resource =resourceLoader.getResource("classpath:GeoLite2-Country.mmdb");
   InputStream dbAsStream = resource.getInputStream();

And copy the InputStream to temp file using Files.copy

Files.copy(inputStream, outputFile.toPath(), StandardCopyOption.REPLACE_EXISTING);

Answer 3

You can use the jackson-databind library.

The Jackson ObjectMapper class ( com.fasterxml.jackson.databind.ObjectMapper ) is one of the simplest way to parse JSON . The Jackson ObjectMapper can parse JSON from a string, stream or file, and create a Java object or object graph representing the parsed JSON. Parsing JSON into Java objects is also referred to as to deserialize Java objects from JSON.

// create Object Mapper
ObjectMapper mapper = new ObjectMapper();

// read JSON file and map/convert to java POJO
try {
    SomeClass someClassObject = mapper.readValue(new File("../src/main/resources/data.json"), SomeClass.class);
    System.out.println(someClassObject);
} catch (IOException e) {
    e.printStackTrace();
}

and you shoud have jackson-databind in your .pom file:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.11.4</version>
</dependency>