interview questions

1.java中为何会出现多线程？

Java内存模型决定了 CPU 不能 彻底利用，为了充分利用CPU，因此产生了多线程技术。

2.多线程中，若是不调用start方法，直接调用run方法会发生什么？

只有调用Thread的start方法，将线程交由JVM控制，才能产生多线程，而直接调用run方法只是一个普通的单线程程式。 执行的是main主线程。

3.为何会出现容器技术？

资源独立、隔离  ； 环境的一致性  ； 轻量化  ； Build Once, Run Everywhere 

4.若是java程序jvm内存分配很大会发生什么？

jvm的内存太小会致使频繁GC，过大会致使GC时间过长。内存越大，JVM 进行 Full GC 所需的时间越久，因为 Full GC 时 stop whole world 的 ，增长请求响应延迟。

5.IO和NIO有什么区别?

Java BIO ： 同步并阻塞，服务器实现模式为一个链接一个线程，即客户端有链接请求时服务器端就须要启动一个线程进行处理，若是这个链接不作任何事情会形成没必要要的线程开销，固然能够经过线程池机制改善。

Java NIO ： 同步非阻塞，服务器实现模式为一个请求一个线程，即客户端发送的链接请求都会注册到多路复用器上，多路复用器轮询到链接有I/O请求时才启动一个线程进行处理。

Java AIO(NIO.2) ： 异步非阻塞，服务器实现模式为一个有效请求一个线程，客户端的I/O请求都是由OS先完成了再通知服务器应用去启动线程进行处理，

6.什么是微服务，你是怎么理解的？

微服务的所涉及的内容很是广，包括基础设施，也包括开发框架，开发、测试、运维都涵盖到了，它毫不是一款普通的架构。从开发框架来说，咱们应该灵活地选择最合适的编程语言和框架，从而实现具体的业务细节，而不要拘泥于在某一种技术上。

7.spring中@Transactional 的事务隔离级别有几种？

DEFAULT :    数据库的默认隔离级别

READ_UNCOMMITTED ： dirty reads, non-repeatable reads and phantom reads
     * can occur.便可能出现脏读，幻读，不可重复读

READ_COMMITTED ： dirty reads are prevented; non-repeatable reads
     * and phantom reads can occur.便可能出现幻读，不可重复读

REPEATABLE_READ ：dirty reads and non-repeatable reads are
     * prevented; phantom reads can occur. 便可能出现幻读

SERIALIZABLE ：dirty reads, non-repeatable reads and phantom
     * reads are prevented. 以上误读状况都被禁止

/**
 * Enumeration that represents transaction isolation levels for use
 * with the {@link Transactional} annotation, corresponding to the
 * {@link TransactionDefinition} interface.
 *
 * @author Colin Sampaleanu
 * @author Juergen Hoeller
 * @since 1.2
 */
public enum Isolation {

	/**
	 * Use the default isolation level of the underlying datastore.
	 * All other levels correspond to the JDBC isolation levels.
	 * @see java.sql.Connection
	 */
	DEFAULT(TransactionDefinition.ISOLATION_DEFAULT),

	/**
	 * A constant indicating that dirty reads, non-repeatable reads and phantom reads
	 * can occur. This level allows a row changed by one transaction to be read by
	 * another transaction before any changes in that row have been committed
	 * (a "dirty read"). If any of the changes are rolled back, the second
	 * transaction will have retrieved an invalid row.
	 * @see java.sql.Connection#TRANSACTION_READ_UNCOMMITTED
	 */
	READ_UNCOMMITTED(TransactionDefinition.ISOLATION_READ_UNCOMMITTED),

	/**
	 * A constant indicating that dirty reads are prevented; non-repeatable reads
	 * and phantom reads can occur. This level only prohibits a transaction
	 * from reading a row with uncommitted changes in it.
	 * @see java.sql.Connection#TRANSACTION_READ_COMMITTED
	 */
	READ_COMMITTED(TransactionDefinition.ISOLATION_READ_COMMITTED),

	/**
	 * A constant indicating that dirty reads and non-repeatable reads are
	 * prevented; phantom reads can occur. This level prohibits a transaction
	 * from reading a row with uncommitted changes in it, and it also prohibits
	 * the situation where one transaction reads a row, a second transaction
	 * alters the row, and the first transaction rereads the row, getting
	 * different values the second time (a "non-repeatable read").
	 * @see java.sql.Connection#TRANSACTION_REPEATABLE_READ
	 */
	REPEATABLE_READ(TransactionDefinition.ISOLATION_REPEATABLE_READ),

	/**
	 * A constant indicating that dirty reads, non-repeatable reads and phantom
	 * reads are prevented. This level includes the prohibitions in
	 * {@code ISOLATION_REPEATABLE_READ} and further prohibits the situation
	 * where one transaction reads all rows that satisfy a {@code WHERE}
	 * condition, a second transaction inserts a row that satisfies that
	 * {@code WHERE} condition, and the first transaction rereads for the
	 * same condition, retrieving the additional "phantom" row in the second read.
	 * @see java.sql.Connection#TRANSACTION_SERIALIZABLE
	 */
	SERIALIZABLE(TransactionDefinition.ISOLATION_SERIALIZABLE);


	private final int value;


	Isolation(int value) { this.value = value; }

	public int value() { return this.value; }

}

8.spring中用到了那些设计模式，请举例说明？

工厂，动态代理（AOP），模板、装饰、单例、适配器等。如bean实例化预留的钩子可经过动态代理实现。

9.mybatis中$和#有什么区别？

#能够防止sql注入 $直接拼接值到sql语句中不安全  但 表名做为变量时，必须使用 ${ }