Spring data Jpa,Mybatis,讀寫鎖,@Lock 使用

Spring data jpa 支持註解式的讀寫鎖(悲觀鎖),實際上這個東西硬編碼也簡單,可是基於Jpa 命名方式定義的Sql,只能用註解添加支持讀寫鎖了,

不瞭解讀寫鎖的能夠點這裏 

mysql讀寫鎖及事務

而且推薦

PESSIMISTIC_READ,
PESSIMISTIC_WRITE,
而不是

READ,
WRITE,
可是官方文檔貌似沒有更新這個案例,踩了一些坑.

新建一個實體Book.java

/**
 * User: laizhenwei
 * Date: 2018-04-18 Time: 9:04
 * Description:
 */
@Entity
@Table(name = "test_book")
@Alias("Book")
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
public class Book extends AbstractJbatisIdEntity{

    private static final long serialVersionUID = -1L;

    private String name;

    private String author;

}

BookRepository.java

/**
 * User: laizhenwei
 * Date: 2018-04-18 Time: 9:11
 * Description:
 */
public interface BookRepository extends JpaRepository<Book,String> {
    @Lock(LockModeType.PESSIMISTIC_READ)
    Book findTop1ByName(String name);
}

BookServiceImpl TimeUnit.SECONDS.sleep(20); 是爲了讓事務延遲提交,好測試save操做須要阻塞到讀寫釋放才能提交

    public static final  CountDownLatch readCount = new CountDownLatch(1);
    public static final  CountDownLatch saveCount = new CountDownLatch(1);

    @Transactional(propagation = Propagation.REQUIRES_NEW)
    @Override
    public Book save(Book book){
        Book book1 = null;
        try {
            readCount.await();
            book1 = getRepository().save(book);
            saveCount.countDown();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return book1;
    }

    @Transactional(propagation = Propagation.REQUIRES_NEW)
    @Override
    public Book findTop1ByName(String name){
        Book book = getRepository().findTop1ByName(name);
        try {
            readCount.countDown();
            TimeUnit.SECONDS.sleep(20);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return book;
    }

JunitTest 先添加一條數據,待會要鎖這個數據

    @Test
    public void save(){
        Book book = new Book();
        book.setName("百年孤獨");
        book.setAuthor("加西亞·馬爾克斯");
        bookRepository.save(book);
    }

開啓兩條線程,一條先加上讀鎖,而後睡眠一會,另外一條線程去修改這個對象的時候,須要阻塞到讀事務提交之後纔會成功

第二個查詢動做不會阻塞,由於讀鎖只對寫操做限制(這裏用直接用bookRepository,是爲了不CountDownLatch 再一次阻塞而已)

    @Test
    @Transactional
    public void findByName() throws InterruptedException {

        new Thread(()->bookService.findTop1ByName("百年孤獨")).start();
        BookServiceImpl.readCount.await();
        Book book =bookRepository.findTop1ByName("百年孤獨");

        book.setAuthor("加西亞·馬爾克斯5");
        new Thread(()->bookService.save(book)).start();
        BookServiceImpl.saveCount.await();

    }

有個有趣的現象,若是直接運行第二次,會發現不用阻塞,就能save成功,由於數據並無作任何修改.

再註釋掉@Lock跑一次,修改 book.setAuthor("加西亞·馬爾克斯5");再保存也不須要等待.

 

Mybatis下的實現,就是手動編碼而已

/**
 * User: laizhenwei
 * Date: 2018-04-18 Time: 9:12
 */
@Mapper
public interface BookMapper extends BaseMapper<Book> {

    @Select("select * from test_book where name=#{name} limit 1 lock in share mode")
    Book findTop1ByName(String name);

}

Service  TimeUnit.SECONDS.sleep(20); 是爲了讓事務延遲提交,好測試save操做須要阻塞到讀寫釋放才能提交

    @Transactional(propagation = Propagation.REQUIRES_NEW)
    @Override
    public Book mapperFindTop1ByName(String name){
        Book book = getMapper().findTop1ByName(name);
        try {
            readCount.countDown();
            TimeUnit.SECONDS.sleep(20);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return book;
    }

JunitTest (這裏用直接用bookRepository,是爲了不CountDownLatch 再一次阻塞而已)

    @Test
    @Transactional
    public void findByName() throws InterruptedException {

        new Thread(()->bookService.mapperFindTop1ByName("百年孤獨")).start();
        BookServiceImpl.readCount.await();
        Book book =bookRepository.findTop1ByName("百年孤獨"); 

        book.setAuthor("加西亞·馬爾克斯3");
        new Thread(()->bookService.save(book)).start();
        BookServiceImpl.saveCount.await();

    }

測試效果與Jpa同樣.