一、普通main代码里使用

1.假设有如下结构的代码

（1）、main方法里复制resource下的文件

import org.apache.commons.io.FileUtils;
import java.io.*;

public class App {
    public static void main(String[] args) throws Exception {
	        //从resources下读取diff2html.min.css为输入流
            InputStream inputStream = App.class.getClassLoader().getResourceAsStream("diff2html.min.css");
            //利用Apache Commons IO库把输入流写入到D:\myExcel\jsAndCss\diff2html.min.css ，如果文件不存在会自动创建
            FileUtils.copyInputStreamToFile(inputStream, new File("D:\\myExcel\\jsAndCss\\diff2html.min.css"));
	}
}

可以把代码和resource目录打成一个jar包也同样适用。运行jar包的时候就会把jar包里resource下的diff2html.min.css文件复制到D:\myExcel\jsAndCss\diff2html.min.css

（2）、main方法里读取resource下的文件

import java.io.*;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class App {
    public static void main(String[] args) throws Exception {
	    List<String> list = new ArrayList<>();
		//从resources下读取diff2html.min.css为输入流
        InputStream inputStream = App.class.getClassLoader().getResourceAsStream("diff2html.min.css");
        BufferedInputStream buf = new BufferedInputStream(inputStream); 
        int len = 0;
        byte[] bys = new byte[1024];
        while ((len = buf.read(bys)) != -1) {
            //diff2html.min.css里每一行的内容添加到list里
			String row = new String(bys, 0, len, StandardCharsets.UTF_8);
			System.out.println(row);
            list.add(row);
        }
        inputStream.close(); 
        buf.close();
    }
}

2.假设有如下结构的代码

import com.haitangxt.utils.ReadSource;

public class App {
    public static void main(String[] args) throws Exception {
            ReadSource readSource = new ReadSource();
        	readSource.copyfile("D:\\myExcel\\jsAndCss");
      }
 }

import org.apache.commons.io.FileUtils;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;

public class ReadSource {
    public  void copyfile(String jsCssPath) {
        try {
        	//从resources下的test/diff2html.min.css读取为输入流
            InputStream inputStream = ReadSource.class.getClassLoader().getResourceAsStream("test/diff2html.min.css");
            //利用Apache Commons IO库把输入流写入到D:\myExcel\jsAndCss\diff2html.min.css ，如果文件不存在会自动创建
            FileUtils.copyInputStreamToFile(inputStream, new File(jsCssPath + "\\diff2html.min.css"));
        } catch (IOException e) {
        	e.printStackTrace();
        }
    }
}

二、对于springboot项目读取resource下的资源文件

@Component
public class WorldAddressUtils {

 public JSONObject getAllCountrys() {
        try (
                //读取文件为字节流
                InputStream file = this.getClass().getResourceAsStream("/resourcesfile/worldAddress.txt");
                //字节流转化为字符流，以UTF-8读取防止中文乱码
                InputStreamReader in = new InputStreamReader(file, "UTF-8");
                //加入到缓存
                BufferedReader buf = new BufferedReader(in);
        ) {
            String str = "";
            while ((str = buf.readLine()) != null) {
				System.out.println(str);
			}
		}catch (IOException e) {
            e.printStackTrace();
        }
    }
}