public class Main{
    public static void main(String[] args) {
        ClassLoader loader = Thread.currentThread().getContextClassLoader();
        System.out.println(loader);
        System.out.println(loader.getParent());
        System.out.println(loader.getParent().getParent());
    }
}

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.boot.jdbc.DataSourceBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.jdbc.core.JdbcTemplate;

import javax.sql.DataSource;

/**
 * @author Song Wei
 * @description
 * @date 12/03/2018 14:47
 */
@Configuration
public class DBConfig {

    @Bean(name = "mysqlDB")
    @Primary
    @ConfigurationProperties(prefix = "second.datasource")
    public DataSource mysqlDataSource() {
        return DataSourceBuilder.create().build();
    }
    @Primary
    @Bean(name = "mysqlJdbcTemplate")
    @Autowired
    public JdbcTemplate jdbcTemplate(@Qualifier("mysqlDB") DataSource dsMySQL) {
        return new JdbcTemplate(dsMySQL);
    }

    @Bean(name = "postgresDB")
    @ConfigurationProperties(prefix = "first.datasource")
    public DataSource postgresDataSource() {
        return  DataSourceBuilder.create().build();
    }

    @Bean(name = "postgresJdbcTemplate")
    @Autowired
    public JdbcTemplate postgresJdbcTemplate(@Qualifier("postgresDB") DataSource dsPostgres) {
        return new JdbcTemplate(dsPostgres);
    }
}

package com.example.demo.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.HashMap;
import java.util.Map;

/**
 * @author Song Wei
 * @description
 * @date 12/03/2018 14:59
 */
@RestController
public class DemoController {

    @Autowired
    @Qualifier("postgresJdbcTemplate")
    private JdbcTemplate postgresTemplate;

    @Autowired
    @Qualifier("mysqlJdbcTemplate")
    private JdbcTemplate mysqlTemplate;

    @GetMapping(value = "/getPostgresUser")
    public String getPostgreUser(){
        Map<String, Object> map = new HashMap();
        String query = "select * from usermaster";
        try {
            map = postgresTemplate.queryForMap(query);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return "PostgreSQL : " + map.toString();
    }
    @RequestMapping(value = "/getMysqlUser")
    public String getMysqlUser() {
        Map<String, Object> map = new HashMap();
        String query = "select * from usermaster";
        try {
            map = mysqlTemplate.queryForMap(query);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return "MySQL : " + map.toString();
    }
}

Object object = Null; //wrong
Object object = null; //ok

private static Object object;
public static void main(String args[]){
    System.out.print("The value of object is:" + object);
}

String s = null; //ok
Float f = null; //ok
Object o = null; //ok

String s = (String)null; //ok
Float f = (Float)null; //ok
Object o = (Object)null; //ok

int i = null; //wrong
float f = null; //wrong
double d = null; //wrong

Integer i = null; //ok
int j = i; //ok

import java.util.HashMap;
import java.util.Map;

public class Test {
    public static void main(String args[]) throws InterruptedException {
      Map numberAndCount = new HashMap<>();
      int[] numbers = {3, 5, 7,9, 11, 13, 17, 19, 2, 3, 5, 33, 12, 5};

      for(int i : numbers){
         int count = numberAndCount.get(i);
         numberAndCount.put(i, count++); // NullPointerException here
      }       
    }
}

public class Main{
    public static void main(String[] args){
        Integer i = null;
        if(i instanceof Integer){
            System.out.println("i is instance of Integer");

        }else{
            System.out.println("i is not an instance of Integer");
        }
    }
}

public class Main{
    public static void main(String[] args){
        Main m = null;
        m.staticMethod();
        m.noneStaticMethod();
    }
    private static void staticMethod(){
        System.out.println("static can be called by null reference");
    }
    private void noneStaticMethod(){
        System.out.println("none static can not be called by null reference");
    }
}

public class Main{
    public static void main(String[] args) {
        Object object1 = null;
        Object object2 = null;
        if (object1 == object2){
            System.out.println("null == null is true");
        }else {
            System.out.println("null == null is false");
        }
    }
}

@Controller
public class FileController {
    //自定义文件处理服务类
    @Autowired
    private FileServiceImpl fileService;
    /**
     *文件上传
     */
    @RequestMapping(value = "/upload",method = RequestMethod.POST)
    public String upload(@RequestParam("file") MultipartFile file) {
        return fileService.uploadFile(file);
    }
}

@Service
public class FileService{
	
	/**
	 * 文件上传处理函数
	 */
	public String uploadFile(MultipartFile file) {
		//判断文件是否为空
		if (file.isEmpty()) {
			return "Empty";
		}
		//文件名
		String fileName = file.getOriginalFilename();
		//文件的保存路径 自定义可单独写个类
		String filePath = "/Users/Songie/";
		//文件最终的存储位置
		File dest = new File(filePath + fileName);
		//判断目标文件所在的目录是否存在
		if(!dest.getParentFile().exists()) {
		    //如果目标文件所在的目录不存在，则创建父目录
		    System.out.println("文件所处目录不存在，创建");
		    //创建目录
		    if(!dest.getParentFile().mkdirs()) {
		        System.out.println("创建文件所处目录失败！");
		        return "fail";
		    }
		}
		//如果文件所在目录存在
		try {
		    //存储文
		    file.transferTo(dest);
		    return "success";
		} catch (IllegalStateException e) {
		    e.printStackTrace();
		} catch (IOException e) {
		    e.printStackTrace();
		}
		return "fail";
	}
}

/**
 * 文件下载
 */
@RequestMapping("/download/{fileName}")
public String downloadFile(@PathVariable("fileName") String fileName,HttpServletRequest request, HttpServletResponse response){
    return fileService.downloadFile(fileName,request,response);
}

/**
 * 文件下载处理函数
 */
public String downloadFile(fileName,HttpServletRequest request, HttpServletResponse response){
	//文件的保存路径 自定义可单独写个类
	String filePath = "/Users/Songie/";
	File file = new File(filePath, fileName);
	//如果文件存在
	if (file.exists()) {
		//重置buffer
		response.resetBuffer();
		//设定编码为UTF-8
		response.setCharacterEncoding("UTF-8");
		//设置头部为下载信息
		response.setHeader("Content-type","application/force-download;charset=UTF-8");
		try {
			response.setHeader("content-disposition", "attachment;filename=" + URLEncoder.encode(fileName, "UTF-8"));// 设置文件名
		} catch (UnsupportedEncodingException e) {
			e.printStackTrace();
		}
		//各种流信息
		FileInputStream fileInputStream = null;
		BufferedInputStream bufferedInputStream = null;
		OutputStream outputStream = null;
		
		try {
			byte[] buffer = new byte[1024];
			fileInputStream = new FileInputStream(file);
			bufferedInputStream = new BufferedInputStream(fileInputStream);
			outputStream=response.getOutputStream();
			int i = bufferedInputStream.read(buffer);
			while (i != -1) {
				outputStream.write(buffer, 0, i);
				i = bufferedInputStream.read(buffer);
			}
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			//关闭各种流
			if (outputStream != null){
			    try {
			        outputStream.close();
			    } catch (IOException e) {
			        e.printStackTrace();
			    }
			
			}
			if (bufferedInputStream!= null) {
			    try {
			        bufferedInputStream.close();
			    } catch (IOException e) {
			        e.printStackTrace();
			    }
			}
			if (fileInputStream != null) {
			    try {
			        fileInputStream.close();
			        return "success";
			    } catch (IOException e) {
			        e.printStackTrace();
			    }
			}
		}//finally结束

	}//if结束
	return "fail";
}//downloadFile结束