这篇3万字的Java后端面试总结，面试官看了瑟瑟发抖（三）

#看日志
tail -f xx.log
#看进程
jps   / ps -ef | grep xx     /netstat -tnlp | grep xx
#看磁盘大小
du   /  df
#看内存大小
free

独立运行在自己进程中。

一系列独立服务共同构建起整个系统。

一个服务只关注自己的独立业务。

轻量的通信机制RESTful API。

使用不同语言开发。

全自动部署机制

刚性事务：遵循ACID原则，强一致性。
柔性事务：遵循BASE理论，最终一致性；与刚性事务不同，柔性事务允许一定时间内，不同节点的数据不一致，但要求最终一致。

BASE 是 Basically Available(基本可用)、Soft state(软状态)和 Eventually consistent (最终一致性)三个短语的缩写。BASE理论是对CAP中AP的一个扩展，通过牺牲强一致性来获得可用性，当出现故障允许部分不可用但要保证核心功能可用，允许数据在一段时间内是不一致的，但最终达到一致状态。满足BASE理论的事务，我们称之为“柔性事务”。

关于如何设计划分服务，我觉得可以学习下DDD领域驱动设计，有很好的指导作用。

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface CustomizeCache {

    String key();

    String value();

    long expireTimes() default 120L; //默认过期时间120s

    int semaphoreCount() default Integer.MAX_VALUE;  //默认限制线程并发数
}

@Component
@Aspect
@Slf4j
public class CacheAspect {
    @Autowired
    private RedisTemplate redisTemplate;

    @Pointcut("@annotation(com.lvshen.demo.redis.cache.CustomizeCache)")
    public void cachePointcut() {
    }

    @Around("cachePointcut()")
    public Object doCache(ProceedingJoinPoint point) {
        Object value = null;
        Semaphore semaphore = null;
        MethodSignature signature = (MethodSignature) point.getSignature();

        try {
            //获取方法上注解的类容
            Method method = point.getTarget().getClass().getMethod(signature.getName(), signature.getMethod().getParameterTypes());
            CustomizeCache annotation = method.getAnnotation(CustomizeCache.class);
            String keyEl = annotation.key();
            String prefix = annotation.value();
            long expireTimes = annotation.expireTimes();
            int semaphoreCount = annotation.semaphoreCount();

            //解析SpringEL表达式
            SpelExpressionParser parser = new SpelExpressionParser();
            Expression expression = parser.parseExpression(keyEl);
            StandardEvaluationContext context = new StandardEvaluationContext();

            //添加参数
            Object[] args = point.getArgs();
            DefaultParameterNameDiscoverer discoverer = new DefaultParameterNameDiscoverer();
            String[] parameterNames = discoverer.getParameterNames(method);
            for (int i = 0; i < parameterNames.length; i++) {
                context.setVariable(parameterNames[i], args[i].toString());
            }

            //解析
            String key = prefix + "::" + expression.getValue(context).toString();

            //判断缓存中是否存在
            value = redisTemplate.opsForValue().get(key);
            if (value != null) {
                log.info("从缓存中读取到值：{}", value);
                return value;
            }

            //自定义组件，如：限流，降级。。。
            //创建限流令牌
            semaphore = new Semaphore(semaphoreCount);
            boolean tryAcquire = semaphore.tryAcquire(3000L, TimeUnit.MILLISECONDS);
            if (!tryAcquire) {
                //log.info("当前线程【{}】获取令牌失败,等带其他线程释放令牌", Thread.currentThread().getName());
                throw new RuntimeException(String.format("当前线程【%s】获取令牌失败,等带其他线程释放令牌", Thread.currentThread().getName()));
            }

            //缓存不存在则执行方法
            value = point.proceed();

            //同步value到缓存
            redisTemplate.opsForValue().set(key, value, expireTimes, TimeUnit.SECONDS);


        } catch (Throwable t) {
            t.printStackTrace();
        } finally {
            if (semaphore == null) {
                return value;
            } else {
                semaphore.release();
            }
        }
        return value;
    }
}

@CustomizeCache(value = "member", key = "#name")
public List<Member> listByNameSelfCache(String name) {
    return memberMapper.listByName(name);
}

public interface UserService {
    void addUser(String name, String password);

    void delUser(String name);
}


public class UserServiceImpl implements UserService{
    @Override
    public void addUser(String name, String password) {
        System.out.println("调用addUser()...");
        System.out.println(String.format("参数为：name[%s],password[%s]",name,password));
    }

    @Override
    public void delUser(String name) {
        System.out.println("调用delUser()");
        System.out.println(String.format("参数为：name[%s]",name));
    }
}

public class JdkProxy  implements InvocationHandler {
    //需要代理的目标对象
    private Object target;
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("JDK动态代理,监听开始...");
        Object invoke = method.invoke(target, args);
        System.out.println("JDK动态代理,监听结束...");
        return invoke;
    }

    public Object getJdkProxy(Object targetObject) {
        this.target = targetObject;
        //实例化
        return Proxy.newProxyInstance(targetObject.getClass().getClassLoader(),targetObject.getClass().getInterfaces(),this);
    }
}

@org.junit.Test
public void testJdkProxy() {
    JdkProxy jdkProxy = new JdkProxy();
    UserService userService = (UserService) jdkProxy.getJdkProxy(new UserServiceImpl());

    userService.addUser("lvshen","123456");

}

public class CglibProxy implements MethodInterceptor {
    private Object target;
    @Override
    public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
        System.out.println("CGLIB动态代理,监听开始...");
        Object invoke = method.invoke(target, objects);
        System.out.println("CGLIB动态代理,监听结束...");
        return invoke;
    }

    public Object getCglibProxy(Object target) {
        this.target = target;
        Enhancer enhancer = new Enhancer();
        //指定父类
        enhancer.setSuperclass(target.getClass());

        enhancer.setCallback(this);
        Object result = enhancer.create();
        return result;
    }
}

public class Test {
    @org.junit.Test
    public void testCglibProxy() {
        CglibProxy cglibProxy = new CglibProxy();
        UserService service = (UserService) cglibProxy.getCglibProxy(new UserServiceImpl());
        service.addUser("zhouzhou","654321");
    }
}

    public class HelloWorldApp {
        public static void main(String[] args) {
            XmlBeanFactory factory = new XmlBeanFactory(new ClassPathResource("beans.xml"));
            HelloWorld obj = (HelloWorld) factory.getBean("helloWorld");
            obj.getMessage();
        }
    }

public static void main(String[] args) {
    ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
    HelloWorld obj = (HelloWorld) context.getBean("helloWorld");
    obj.getMessage();
}
   

Select * from emp where name = #{employeeName}

Select * from emp where name = 'Smith'; 

Select * from emp where name = ${employeeName}

Select * from emp where name = Smith

DEBUG [http-nio-8080-exec-5] - ==>  Preparing: select * from user where account = ? and password = ?  DEBUG [http-nio-8080-exec-5] - ==> Parameters: 20200801(String), 111111 or account = 'admin' (String) DEBUG [http-nio-8080-exec-5] - <==      Total: 0 返回结果：null

DEBUG [http-nio-8080-exec-5] - ==>  Preparing: select * from user where account = ? and password = ?  DEBUG [http-nio-8080-exec-5] - ==> Parameters: 201301001(String), 111111 or account = 'admin' (String) DEBUG [http-nio-8080-exec-5] - <==      Total: 0 转换为实际的SQL语句：select * from user where account = '20200801' and password = '111111 or account = 'admin'' 

@Data
public class Employee {
        public string Id ;
        public string Name ;
        public string Sex ;
        public DateTime? BirthDay;
      
        /// 直属上级的Id
        public string ParentId ;
 }

//实现方法略    
public class EmpDAO {
        public static bool AddEmployee(Employee emp);
        public static bool UpdateEmployee(Employee emp);
        public static bool DeleteEmployee(Employee emp);
        public static Employee GetEmployeeById(string Id);
}

@Data
public class Employee {
        public string Id ;
        public string Name ;
        public string Sex ;
        public DateTime;
  
        /// 直属上级的Id
        public string ParentIdl;
        private Employee _parent;

        public static Employee query(string id){
            Employee emp = new Employee();
            //实现略,仅需填充emp的熟悉即可
            return emp;
        }
    
        //保存对象,实现略
        public bool Save() {
            return true;
        }
        
        // 删除对象,实现略
        public bool Drop(){
            return true;
        }
      
    }

public class LazySimpleSingleton {
    private static volatile LazySimpleSingleton instance = null;

    private LazySimpleSingleton(){
        if (instance != null) {
            throw new RuntimeException("该构造方法禁止获取");
        }
    }

    public static LazySimpleSingleton getInstance() {
        if (instance == null) {
            synchronized (LazySimpleSingleton.class) {
                if (instance == null) {
                    instance = new LazySimpleSingleton();
                }
            }
        }
        return instance;
    }
}

public static int getIndex(int[] arr, int key) {
        int mid = arr.length / 2;
        if (arr[mid] == key) {
            return mid;
        }
        int start = 0;
        int end = arr.length - 1;
        while (start <= end) {
            mid = (end - start) / 2 + start;
            if (arr[mid] == key) {
                return mid;
            } else if (arr[mid] > key) {
                end = mid - 1;
            } else {
                start = start + 1;
            }
        }
        //找不到，返回-1
        return -1;
    }