无法为事务打开 JPA EntityManager；嵌套异常是 javax.persistence.PersistenceException

Could not open JPA EntityManager for transaction; nested exception is javax.persistence.PersistenceException

提问by emoleumassi

when i run my code, i get this exception:

当我运行我的代码时，我得到这个异常：

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1121)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:357)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2482)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2519)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2304)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:834)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:416)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:346)
    at java.sql.DriverManager.getConnection(DriverManager.java:664)
    at java.sql.DriverManager.getConnection(DriverManager.java:208)
    at org.springframework.jdbc.datasource.DriverManagerDataSource.getConnectionFromDriverManager(DriverManagerDataSource.java:153)
    at org.springframework.jdbc.datasource.DriverManagerDataSource.getConnectionFromDriver(DriverManagerDataSource.java:144)
    at org.springframework.jdbc.datasource.AbstractDriverBasedDataSource.getConnectionFromDriver(AbstractDriverBasedDataSource.java:155)
    at org.springframework.jdbc.datasource.AbstractDriverBasedDataSource.getConnection(AbstractDriverBasedDataSource.java:120)
    at org.hibernate.engine.jdbc.connections.internal.DatasourceConnectionProviderImpl.getConnection(DatasourceConnectionProviderImpl.java:139)
    at org.hibernate.tool.hbm2ddl.SuppliedConnectionProviderConnectionHelper.prepare(SuppliedConnectionProviderConnectionHelper.java:51)
    at org.hibernate.tool.hbm2ddl.DatabaseExporter.<init>(DatabaseExporter.java:52)
    at org.hibernate.tool.hbm2ddl.SchemaExport.execute(SchemaExport.java:367)
    at org.hibernate.tool.hbm2ddl.SchemaExport.create(SchemaExport.java:304)
    at org.hibernate.tool.hbm2ddl.SchemaExport.create(SchemaExport.java:293)
    at org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:517)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1857)
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
Aug 25, 2015 8:23:45 PM com.sun.jersey.spi.container.ContainerResponse mapMappableContainerException
SCHWERWIEGEND: The RuntimeException could not be mapped to a response, re-throwing to the HTTP container
org.springframework.transaction.CannotCreateTransactionException: Could not open JPA EntityManager for transaction; nested exception is javax.persistence.PersistenceException: org.hibernate.exception.JDBCConnectionException: Could not open connection
    at org.springframework.orm.jpa.JpaTransactionManager.doBegin(JpaTransactionManager.java:431)
    at org.springframework.transaction.support.AbstractPlatformTransactionManager.getTransaction(AbstractPlatformTransactionManager.java:373)
    at org.springframework.transaction.interceptor.TransactionAspectSupport.createTransactionIfNecessary(TransactionAspectSupport.java:457)
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:276)
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:207)
    at com.sun.proxy.$Proxy76.create(Unknown Source)

I don't know where is the problem. But yesterday i worked fine.

我不知道问题出在哪里。但昨天我工作得很好。

my database.properties:

我的database.properties：

jdbc.driverClassName=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/streaming
jdbc.username=root
jdbc.password=password

hibernate.dialect=org.hibernate.dialect.MySQLDialect
hibernate.showsql=true
hibernate.hbm2ddl.auto=create

It is possible for me to run mysql directly from command line. Where is the problem? Was is wrong. I use spring 4 and hibernate with mysql as database language.

我可以直接从命令行运行 mysql。问题出在哪儿？是错的。我使用 spring 4 并使用 mysql 作为数据库语言进行休眠。

Has someone any ideas?

有人有什么想法吗？

Thank.

谢谢。

回答by Amit Tamrakar

There is CommunicationsException: Communications link failure.

有 CommunicationsException: Communications link failure。

If you get a SQLException: Connection refused or Connection timed out or a MySQL specific

如果您收到 SQLException: Connection denied 或 Connection timed out 或 MySQL 特定的异常

CommunicationsException: Communications link failure

then it means that the DB isn't reachable at all. This can have one or more of the following causes:

那么这意味着数据库根本无法访问。这可能有以下一种或多种原因：

• IP address or hostname in JDBC URL is wrong.
• Hostname in JDBC URL is not recognized by local DNS server.
• Port number is missing or wrong in JDBC URL.
• DB server is down.
• DB server doesn't accept TCP/IP connections.
• DB server has run out of connections.
• Something in between Java and DB is blocking connections, e.g. a firewall or proxy.

• JDBC URL 中的 IP 地址或主机名错误。
• 本地 DNS 服务器无法识别 JDBC URL 中的主机名。
• JDBC URL 中的端口号丢失或错误。
• 数据库服务器已关闭。
• 数据库服务器不接受 TCP/IP 连接。
• 数据库服务器的连接用完了。
• Java 和 DB 之间的某些东西正在阻止连接，例如防火墙或代理。

To solve the one or the other, follow the following advices:

要解决其中之一，请遵循以下建议：

• Verify and test them with ping.
• Refresh DNS or use IP address in JDBC URL instead.
• Verify it based on my.cnf of MySQL DB.
• Start the DB.
• Verify if mysqld is started without the --skip-networking option.
• Restart the DB and fix your code accordingly that it closes connections in finally.
• Disable firewall and/or configure firewall/proxy to allow/forward the port.

• 用 ping 验证和测试它们。
• 刷新 DNS 或改用 JDBC URL 中的 IP 地址。
• 根据 MySQL DB 的 my.cnf 进行验证。
• 启动数据库。
• 验证是否在没有 --skip-networking 选项的情况下启动了 mysqld。
• 重新启动数据库并相应地修复您的代码，最终关闭连接。
• 禁用防火墙和/或配置防火墙/代理以允许/转发端口。

Source: More Details

资料来源：更多详情

回答by ACV

DB is down. You must start the DB server and check connectivity with either telnet or mysql command line client first.

数据库挂了。您必须首先启动数据库服务器并检查与 telnet 或 mysql 命令行客户端的连接。

回答by rbansal

I was not unable to figure out the exact cause, but I realized that it was because of some thread pools since I was able to connect to the db using the terminal. for checking the connectivity you can use the below command.

我无法找出确切原因，但我意识到这是因为一些线程池，因为我能够使用终端连接到数据库。要检查连接，您可以使用以下命令。

psql -h hostname -U username -d database

so restarting the server worked for me.

所以重新启动服务器对我有用。