一般获取工程resource的方式

1、首先拿到工程根目录
String rootPath = System.getProperty("user.dir") // D:\temp\repo\demo
2、拼接具体的文件相对路径地址
/src/main/resources/demo.txt

这种一般的方式在普通的maven or gradle工程中运行是没有问题的，但工程一旦打包之后作为jar包依赖，该"user.dir"拿到的不再是打包工程的根目录，无法获取打包工程相应的资源文件。
如果是在Android Studio插件中，拿到的rootPath就是运行时的"user.dir"了，显然这是不正确的，那么就插件的相关功能就无法使用。
JVM启动时通过执行本地方法自动初始化了这个系统属性，"user.dir"代表用户当前的工作目录。

文件流的包装玩法

stream结尾都是采用字节流，reader和writer结尾都是采用字符流
常见的字符输入流用途与区别
（1）Reader：是字符流的抽象基类，它包含了重要方法有read和close
（2）InputStreamReader：可以把InputStream中的字节数据流根据字符编码方式转换成字符数据流。
（3）FileReader：可以把FileInputStream中的字节数据转换成根据字符编码方式转成字符数字流。
（4）BufferedReader：可以把字符输入流进行封装，将数据进行缓冲，提高读取效率。它含有read（末尾返回-1）和readLine()（末尾返回null)。

通过classloader获取工程resource的方式

这里不叙述原理，直接上代码，
import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.JarURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**

• jar中读取resource内容与常规读取disk内容

• @since 2020-10-09
  */
  public class TestJar {
  private static final Logger LOGGER = LoggerFactory.getLogger(TestJar.class);

  /**

  • <h2>scan file in jar by resSubdirectory</h2>
  • @param resSubdirectory resSubdirectory, like "config" not "resource/config"
  • @return filePaths
    */
    public static List<String> scanFilePathsInJar(String resSubdirectory) {
    List<String> filePaths = new ArrayList<>();
    URL url = TestJar.class.getResource(File.separator + resSubdirectory);
    if (url.getPath().contains(".jar!")) {
    try {
    if (url.openConnection() instanceof JarURLConnection) {
    JarURLConnection jarURLConnection = (JarURLConnection) url.openConnection();
    JarFile jarFile = jarURLConnection.getJarFile();
    for (Enumeration<JarEntry> enumeration = jarFile.entries(); enumeration.hasMoreElements();) {
    JarEntry jarEntry = enumeration.nextElement();
    if (!jarEntry.isDirectory() && jarEntry.getName().startsWith(resSubdirectory)) {
    filePaths.add(File.separator + jarEntry.getName());
    }
    }
    }
    } catch (IOException e) {
    LOGGER.error("Failed to scan jar!");
    }
    }
    return filePaths;
    }

  /**

  • <h2>read file text from resource</h2>
  • @param subdirectory resource subdirectory path, under the "/src/main/resources/" like "/config"
  • @param resourceName resource file name
  • @return file text
    */
    public String readFileText(String subdirectory, String resourceName) {
    String resourcePath = subdirectory + resourceName;
    final StringBuilder sb = new StringBuilder();
    try (InputStream is = this.getClass().getResourceAsStream(resourcePath)) {
    byte[] bytes = new byte[2048];
    int n;
    while ((n = is.read(bytes, 0, bytes.length)) != -1) {
    String str = new String(bytes, 0, n, StandardCharsets.UTF_8);
    sb.append(str);
    }
    } catch (IOException e) {
    LOGGER.error("File read failed!");
    }
    return sb.toString();
    }

  /**

  • <h2>read file text from disk</h2>
  • @param file file absolute path
  • @return file text
    */
    public String readFileText(File file) {
    final StringBuilder sb = new StringBuilder();
    try (BufferedInputStream in = new BufferedInputStream(new FileInputStream(file))) {
    byte[] bytes = new byte[2048];
    int n;
    while ((n = in.read(bytes, 0, bytes.length)) != -1) {
    String str = new String(bytes, 0, n, StandardCharsets.UTF_8);
    LOGGER.info(str);
    sb.append(str);
    }
    } catch (IOException e) {
    LOGGER.error("File read failed!");
    }
    return sb.toString();
    }

  /**

  • <h2>read section text from jar</h2>
  • @param filePath jar inner file path
  • @param sectionFlag section flag
  • @return section text
    */
    public String readSectionFromJar(String filePath, String sectionFlag) {
    try (InputStream inputStream = this.getClass().getResourceAsStream(filePath)) {
    return getSectionText(sectionFlag, inputStream);
    } catch (IOException e) {
    LOGGER.error("Section read failed!");
    }
    return "";
    }

  /**

  • <h2>read section text from disk</h2>
  • @param filePath disk file absolute path
  • @param sectionFlag section flag
  • @return section text
    */
    public String readSectionFromDisk(String filePath, String sectionFlag) {
    try (InputStream inputStream = new FileInputStream(new File(filePath))) {
    return getSectionText(sectionFlag, inputStream);
    } catch (IOException e) {
    LOGGER.error("Section read failed!");
    }
    return "";
    }

  private String getSectionText(String sectionFlag, InputStream inputStream) {
  final StringBuilder sb = new StringBuilder();
  try (BufferedReader bufferedReader =
  new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8))) {
  String str;
  boolean flag = false;
  while ((str = bufferedReader.readLine()) != null) {
  if (str.contains(sectionFlag)) {
  flag = true;
  continue;
  }
  if (str.contains("##END")) { // hard code
  flag = false;
  continue;
  }
  if (flag) {
  sb.append(str).append(System.lineSeparator()); // "\r\n"
  }
  }
  } catch (IOException e) {
  LOGGER.error("Get file section failed!");
  }
  return deleteStrEndLineSeparator(sb.toString());
  }

  private String deleteStrEndLineSeparator(String str) {
  return str != null && !str.isEmpty() ? str.substring(0, str.length() - System.lineSeparator().length()) : "";
  }
  }

参考

https://www.cnblogs.com/Hermioner/p/9773188.html
https://www.jianshu.com/p/28693aad491b
https://blog.csdn.net/b_h_l/article/details/7767829
https://my.oschina.net/u/3505620/blog/3275265