Java – 从资源文件夹中读取文件

分手后的思念是犯贱 2023-09-26 09:36 18阅读 0赞

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。

![format_png][]

`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。

![b9a57fc5f3274c0eb67e34db22e1bba4.webp][]

**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。

![format_png][]

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”][jar]中的指南

对于 **Maven**，我们应该使用**在 pom** 中定义的 [maven-jar 插件][maven-jar].xml：

<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。

![format_png 1][]

**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][https_github.com_favtuts_java-core-tutorials-examples]
> 
> $ cd java-io

[format_png]: https://img-blog.csdnimg.cn/img_convert/b718c2966936e7b5621a9119e4e64313.webp?x-oss-process=image/format,png
[b9a57fc5f3274c0eb67e34db22e1bba4.webp]: https://img-blog.csdnimg.cn/b9a57fc5f3274c0eb67e34db22e1bba4.webp
[jar]: https://stackoverflow.com/questions/9689793/cant-execute-jar-file-no-main-manifest-attribute
[maven-jar]: https://maven.apache.org/plugins/maven-jar-plugin/
[format_png 1]: https://img-blog.csdnimg.cn/img_convert/e911980e05e3da248b2abb03177e3158.webp?x-oss-process=image/format,png
[https_github.com_favtuts_java-core-tutorials-examples]: https://github.com/favtuts/java-core-tutorials-examples