在Java中，我们可以使用getResourceAsStream或getResource从resources文件夹或类路径的根读取一个或多个文件。

JAVA不断读取文件夹下文件 java读取文件夹中的文件_java

getResourceAsStream方法返回一个InputStream .




// the stream holding the file content
  InputStream is = getClass().getClassLoader().getResourceAsStream("file.txt");

  // for static access, uses the class name directly
  InputStream is = JavaClassName.class.getClassLoader().getResourceAsStream("file.txt");



getResource方法返回 一个URL，通常将其转换为File ;在 JAR 文件中不起作用。




// get the file url, not working in JAR file.
  URL resource = getClass().getClassLoader().getResource("file.txt");
  if (resource == null) {
      throw new IllegalArgumentException("file not found!");
  } else {

      // failed if files have whitespaces or special characters
      //return new File(resource.getFile());

      return new File(resource.toURI());
  }

  // for static access
  // URL resource = JavaClassName.class.getClassLoader().getResource("fileName");



1. 资源文件夹中的文件

1.1 查看src/main/resources中的文件，稍后我们将访问这些文件并打印出文件内容。

JAVA不断读取文件夹下文件 java读取文件夹中的文件_json_02

src/main/resources/database.properties




datasource.url=jdbc:mysql://localhost/favtuts?useSSL=false
datasource.username=root
datasource.password=password
datasource.driver-class-name=com.mysql.jdbc.Driver



src/main/resources/json/file1.json




{
  "name": "favtuts",
  "age": 38
}



src/main/resources/json/file2.json




{
  "name": "jack",
  "age": 40
}



src/main/resources/json/sub/subfile1.json




{
  "name": "sub",
  "age": 99
}



1.2 默认情况下，构建工具（如 Maven、Gradle 或常见的 Java 实践）会将所有文件从src/main/resources复制到target/classes或build/classes的根目录。因此，当我们尝试从src/main/resources读取文件时，我们从项目类路径的根读取该文件。

JAVA不断读取文件夹下文件 java读取文件夹中的文件_java

1.3 下面是一个 JAR 文件结构。通常，resources文件夹中的文件将复制到类路径的根目录。

$ jar -tf target/example.jar
META-INF/MANIFEST.MF
META-INF/
json/
json/sub/
json/file2.json
json/sub/subfile1.json
json/file1.json
database.properties
com/
com/favtuts/
com/favtuts/io/
com/favtuts/io/utils/
//...

2. 从资源文件夹中获取文件。

2.1 下面的示例演示如何使用getResourceAsStream和getResource方法从resources文件夹中读取json/file1.json文件并打印出文件内容。

注意

  • getResource方法在 JAR 文件中不起作用。
  • getResourceAsStream方法在任何地方都有效。

FileResourcesUtils.java




package com.favtuts.io.utils;

import java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URISyntaxException;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.util.List;

public class FileResourcesUtils {

    public static void main(String[] args) throws Exception {

        FileResourcesUtils app = new FileResourcesUtils();

        //String fileName = "database.properties";
        String fileName = "json/file1.json";

        System.out.println("getResourceAsStream : " + fileName);
        InputStream is = app.getFileFromResourceAsStream(fileName);
        printInputStream(is);

        System.out.println("\ngetResource : " + fileName);
        File file = app.getFileFromResource(fileName);
        printFile(file);

    }

    // get a file from the resources folder
    // works everywhere, IDEA, unit test and JAR file.
    private InputStream getFileFromResourceAsStream(String fileName) {

        // The class loader that loaded the class
        ClassLoader classLoader = getClass().getClassLoader();
        InputStream inputStream = classLoader.getResourceAsStream(fileName);

        // the stream holding the file content
        if (inputStream == null) {
            throw new IllegalArgumentException("file not found! " + fileName);
        } else {
            return inputStream;
        }

    }

    /*
        The resource URL is not working in the JAR
        If we try to access a file that is inside a JAR,
        It throws NoSuchFileException (linux), InvalidPathException (Windows)

        Resource URL Sample: file:java-io.jar!/json/file1.json
     */
    private File getFileFromResource(String fileName) throws URISyntaxException{

        ClassLoader classLoader = getClass().getClassLoader();
        URL resource = classLoader.getResource(fileName);
        if (resource == null) {
            throw new IllegalArgumentException("file not found! " + fileName);
        } else {

            // failed if files have whitespaces or special characters
            //return new File(resource.getFile());

            return new File(resource.toURI());
        }

    }

    // print input stream
    private static void printInputStream(InputStream is) {

        try (InputStreamReader streamReader =
                    new InputStreamReader(is, StandardCharsets.UTF_8);
             BufferedReader reader = new BufferedReader(streamReader)) {

            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    // print a file
    private static void printFile(File file) {

        List<String> lines;
        try {
            lines = Files.readAllLines(file.toPath(), StandardCharsets.UTF_8);
            lines.forEach(System.out::println);
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}



输出

getResourceAsStream : json/file1.json
{
"name": "favtuts",
"age": 38
}
getResource : json/file1.json
{
"name": "favtuts",
"age": 38
}

2.2 现在，我们将项目打包到一个JAR文件中并运行它;这一次，getResource将失败并返回NoSuchFileException或InvalidPathException。我们无法通过资源 URL 读取 JAR 文件中的文件。

在 Linux （Ubuntu） 上运行 JAR 文件，它会引发NoSuchFileException。

$ mvn clean package
$ java -jar target/java-io.jar
$ jar tf target/java-io.jar | more
$ java -cp target/java-io.jar com.favtuts.io.utils.FileResourcesUtils
getResourceAsStream : json/file1.json
{
"name": "favtuts",
"age": 38
}
# for new File(resource.getFile());
getResource : json/file1.json
java.nio.file.NoSuchFileException: file:/home/favtuts/projects/core-java/java-io/target/java-io.jar!/json/file1.json
at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:92)
at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:116)
at java.base/sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:219)
at java.base/java.nio.file.Files.newByteChannel(Files.java:370)
at java.base/java.nio.file.Files.newByteChannel(Files.java:421)
at java.base/java.nio.file.spi.FileSystemProvider.newInputStream(FileSystemProvider.java:420)
at java.base/java.nio.file.Files.newInputStream(Files.java:155)
at java.base/java.nio.file.Files.newBufferedReader(Files.java:2838)
at java.base/java.nio.file.Files.readAllLines(Files.java:3329)
at com.favtuts.io.utils.FileResourcesUtils.printFile(FileResourcesUtils.java:135)
at com.favtuts.io.utils.FileResourcesUtils.main(FileResourcesUtils.java:24)
# for new File(resource.toURI());
getResource : json/file1.json
Exception in thread "main" java.lang.IllegalArgumentException: URI is not hierarchical
at java.base/java.io.File.<init>(File.java:420)
at com.favtuts.io.utils.FileResourcesUtils.getFileFromResource(FileResourcesUtils.java:112)
at com.favtuts.io.utils.FileResourcesUtils.main(FileResourcesUtils.java:29)

您可能会收到以下消息

no main manifest attribute, in "java-io.jar"

要解决此问题，请阅读堆栈溢出：无法执行 jar 文件：“无主清单属性”中的指南

对于 Maven，我们应该使用在 pom 中定义的 maven-jar 插件.xml：




<!-- Make this jar executable -->
        <plugin>
          <groupId>org.apache.maven.plugins</groupId>
          <artifactId>maven-jar-plugin</artifactId>
          <version>3.2.0</version>
          <configuration>
              <excludes>
                  <exclude>**/log4j.properties</exclude>
              </excludes>
              <archive>
                  <manifest>
                      <addClasspath>true</addClasspath>                                            
                      <mainClass>com.favtuts.io.utils.FileResourcesUtils</mainClass>                      
                      <classpathPrefix>dependency-jars/</classpathPrefix>
                  </manifest>
              </archive>
          </configuration>
        </plugin>



附言：此示例使用 Maven 插件创建 JAR 文件。

要检查 jar 文件中的主类，请执行以下操作：

$ jar tf target/java-io.jar | more

3. 从资源文件夹获取文件 – 单元测试

3.1 我们将测试资源放在单元测试的src/test/resources文件夹中。通常，测试资源中的文件将复制到target/test-classes文件夹。

JAVA不断读取文件夹下文件 java读取文件夹中的文件_JAVA不断读取文件夹下文件_04

src/test/resources/json/file1.json




{
  "name": "unit test",
  "age": 38
}



src/test/resources/database.properties.properties




datasource.url=jdbc:mysql://localhost/test?useSSL=false
datasource.username=test
datasource.password=password
datasource.driver-class-name=com.mysql.jdbc.Driver



3.2 它的工作方式与我们从src/main/resources中读取文件的方式相同。我们使用相同的getResourceAsStream和getResource方法从 src/test/resources读取文件.

FileResourcesTest.java




package com.favtuts.io;

import java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URISyntaxException;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.util.List;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

// Unit test class
public class FileResourcesTest {

    @DisplayName("Test loading a JSON file")
    @Test
    void loadJSONTest() {

        String fileName = "json/file1.json";

        ClassLoader classLoader = getClass().getClassLoader();

        try (InputStream inputStream = classLoader.getResourceAsStream(fileName);
            InputStreamReader streamReader = new InputStreamReader(inputStream, StandardCharsets.UTF_8);
            BufferedReader reader = new BufferedReader(streamReader)
        ) {
            
            String line;
            while((line = reader.readLine()) != null) {
                System.out.println(line);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }


    @DisplayName("Test loading a properties file")
    @Test
    void loadPropTest() throws IOException, URISyntaxException {

        String fileName = "database.properties";

        ClassLoader classLoader = getClass().getClassLoader();

        URL resource = classLoader.getResource(fileName);
        if (resource == null) {
            throw new IllegalArgumentException("file not found! " + fileName);
        }

        //File file = new File(resource.getFile());
        File file = new File(resource.toURI());

        List<String> lines = Files.readAllLines(file.toPath(), StandardCharsets.UTF_8);
        lines.forEach(System.out::println);
    }
    
}



输出

{
"name": "unit test",
"age": 38
}
datasource.url=jdbc:mysql://localhost/test?useSSL=false
datasource.username=test
datasource.password=password
datasource.driver-class-name=com.mysql.jdbc.Driver

4. 从资源文件夹中获取所有文件。（非 JAR 环境）

如果我们不知道确切的文件名，并且想要读取所有文件，包括资源文件夹中的子文件夹文件，我们可以使用NIO Files.walk轻松访问和读取文件。

4.1 以下示例使用Files.walk从src/main/resources/json文件夹中读取所有文件：

FileResourcesUtils.java


p



package com.favtuts.io.utils;

import java.io.*;
import java.net.URISyntaxException;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;
import java.util.stream.Collectors;

public class FileResourcesUtils {

    public static void main(String[] args) throws IOException {

        FileResourcesUtils app = new FileResourcesUtils();

        // read all files from a resources folder
        try {

            // files from src/main/resources/json
            List<File> result = app.getAllFilesFromResource("json");
            for (File file : result) {
                System.out.println("file : " + file);
                printFile(file);
            }

        } catch (URISyntaxException | IOException e) {
            e.printStackTrace();
        }

    }

    private List<File> getAllFilesFromResource(String folder)
        throws URISyntaxException, IOException {

        ClassLoader classLoader = getClass().getClassLoader();

        URL resource = classLoader.getResource(folder);

        // dun walk the root path, we will walk all the classes
        List<File> collect = Files.walk(Paths.get(resource.toURI()))
                .filter(Files::isRegularFile)
                .map(x -> x.toFile())
                .collect(Collectors.toList());

        return collect;
    }

    // print a file
    private static void printFile(File file) {

        List<String> lines;
        try {
            lines = Files.readAllLines(file.toPath(), StandardCharsets.UTF_8);
            lines.forEach(System.out::println);
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}



输出

file : /home/favtuts/projects/core-java/java-io/target/classes/json/file1.json
{
"name": "favtuts",
"age": 38
}
file : /home/favtuts/projects/core-java/java-io/target/classes/json/file2.json
{
"name": "jack",
"age": 40
}
file : /home/favtuts/projects/core-java/java-io/target/classes/json/sub/subfile1.json
{
"name": "sub",
"age": 99
}

4.2 但是，示例 4.1 中的标准Files.walk无法直接访问 JAR 文件中的文件，请尝试在 JAR 环境中运行示例 4.1，然后它会引发FileSystemNotFoundException。

$ mvn clean package
$ java -jar target/java-io.jar
Exception in thread "main" java.nio.file.FileSystemNotFoundException
at jdk.zipfs/jdk.nio.zipfs.ZipFileSystemProvider.getFileSystem(ZipFileSystemProvider.java:169)
at jdk.zipfs/jdk.nio.zipfs.ZipFileSystemProvider.getPath(ZipFileSystemProvider.java:155)
at java.base/java.nio.file.Path.of(Path.java:208)
at java.base/java.nio.file.Paths.get(Paths.java:97)
at com.favtuts.io.utils.FileResourcesUtils.getAllFilesFromResource(FileResourcesUtils.java:128)
at com.favtuts.io.utils.FileResourcesUtils.main(FileResourcesUtils.java:35)

5. 从资源文件夹中获取所有文件。（JAR 版本）

5.1 此示例演示Files.walk如何通过FileSystems和jar:file:xxx.jar URI读取 JAR 文件内的文件夹 。

这个想法是：

  • 文件使用FileSystems在 JAR 文件中遍历文件夹，并获取所有文件名，请参见getPathsFromResourceJAR()
  • 循环所有文件名，访问并打印每个文件，如示例 2.1，请参阅getFileFromResourceAsStream()

FileResourcesUtils.java




package com.favtuts.io.utils;

import java.io.*;
import java.net.URI;
import java.net.URISyntaxException;
import java.nio.charset.StandardCharsets;
import java.nio.file.FileSystem;
import java.nio.file.FileSystems;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;

public class FileResourcesUtils {

    public static void main(String[] args) throws IOException {

        FileResourcesUtils app = new FileResourcesUtils();

        // Sample 3 - read all files from a resources folder (JAR version)
        try {

            // get paths from src/main/resources/json
            List<Path> result = app.getPathsFromResourceJAR("json");
            for (Path path : result) {
                System.out.println("Path : " + path);

                String filePathInJAR = path.toString();
                // Windows will returns /json/file1.json, cut the first /
                // the correct path should be json/file1.json
                if (filePathInJAR.startsWith("/")) {
                    filePathInJAR = filePathInJAR.substring(1, filePathInJAR.length());
                }

                System.out.println("filePathInJAR : " + filePathInJAR);

                // read a file from resource folder
                InputStream is = app.getFileFromResourceAsStream(filePathInJAR);
                printInputStream(is);
            }

        } catch (URISyntaxException | IOException e) {
            e.printStackTrace();
        }

    }

    // get a file from the resources folder
    // works everywhere, IDEA, unit test and JAR file.
    private InputStream getFileFromResourceAsStream(String fileName) {

        // The class loader that loaded the class
        ClassLoader classLoader = getClass().getClassLoader();
        InputStream inputStream = classLoader.getResourceAsStream(fileName);

        // the stream holding the file content
        if (inputStream == null) {
            throw new IllegalArgumentException("file not found! " + fileName);
        } else {
            return inputStream;
        }

    }

    // Get all paths from a folder that inside the JAR file
    private List<Path> getPathsFromResourceJAR(String folder)
        throws URISyntaxException, IOException {

        List<Path> result;

        // get path of the current running JAR
        String jarPath = getClass().getProtectionDomain()
                .getCodeSource()
                .getLocation()
                .toURI()
                .getPath();
        System.out.println("JAR Path :" + jarPath);

        // file walks JAR
        URI uri = URI.create("jar:file:" + jarPath);
        try (FileSystem fs = FileSystems.newFileSystem(uri, Collections.emptyMap())) {
            result = Files.walk(fs.getPath(folder))
                    .filter(Files::isRegularFile)
                    .collect(Collectors.toList());
        }

        return result;

    }

    // print input stream
    private static void printInputStream(InputStream is) {

        try (InputStreamReader streamReader = new InputStreamReader(is, StandardCharsets.UTF_8);
             BufferedReader reader = new BufferedReader(streamReader)) {

            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}



输出

$ java -jar target/java-io.jar
JAR Path :/C:/Users/favtuts/projects/core-java/java-io/target/java-io.jar
Path : /json/file2.json
filePathInJAR : json/file2.json
{
"name": "jack",
"age": 40
}
Path : /json/file1.json
filePathInJAR : json/file1.json
{
"name": "favtuts",
"age": 38
}
Path : /json/sub/subfile1.json
filePathInJAR : json/sub/subfile1.json
{
"name": "sub",
"age": 99
}

下载源代码

$ git clone https://github.com/favtuts/java-core-tutorials-examples

$ cd java-io