JPA、休眠与 c3p0 和 Postgres。检测数据库连接问题

Question

我正在编写一个应用程序，它通过带有 c3p0 池的 Hibernate 连接到 Postgres DB。在主界面出现之前，我想检测任何有效的数据库连接设置以及与数据库的连接。如果设置无效，我想向用户显示一条消息并建议更改设置或关闭应用程序。 但问题是EntityManagerFactory在连接不成功后不会抛出异常甚至返回。

这是一个错误的连接设置产生错误的代码示例：

public void connect(ConnectionSettingsModel conSet) throws Exception {
    Map<String, String> connectionProperties = new HashMap<>();
    connectionProperties.put("javax.persistence.jdbc.url", conSet.getUrl());
    connectionProperties.put("javax.persistence.jdbc.user", conSet.getUser());
    connectionProperties.put("javax.persistence.jdbc.password", conSet.getPassword());
    connectionProperties.put("hibernate.default_schema", conSet.getSchema());

    System.out.println("Before creating EM");
    EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("postgres-connect", connectionProperties);
    EntityManager entityManager = entityManagerFactory.createEntityManager();
    System.out.println("After creating EM");
 }

persistence.xml中的c3p0配置：

<property name="hibernate.connection.provider_class" value="org.hibernate.connection.C3P0ConnectionProvider"/>
<property name="hibernate.c3p0.min_size" value="0"/>
<property name="hibernate.c3p0.max_size" value="10"/>
<property name="hibernate.c3p0.timeout" value="300"/>
<property name="hibernate.c3p0.idle_test_period" value="3000"/>
<property name="hibernate.c3p0.max_statements" value="50"/>
<property name="hibernate.c3p0.acquireRetryAttempts" value="1"/>

用户名不存在的日志示例：

Before creating EM
14:47:07,009 INFO [com.mchange.v2.log.MLog] - MLog clients using log4j logging.
14:47:07,239 INFO [com.mchange.v2.c3p0.C3P0Registry] - Initializing c3p0-0.9.5.2 [built 08-December-2015 22:06:04 -0800; debug? true; trace: 10]
14:47:07,303 INFO [com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource] - Initializing c3p0 pool... com.mchange.v2.c3p0.PoolBackedDataSource@e945fe41 [ connectionPoolDataSource -> com.mchange.v2.c3p0.WrapperConnectionPoolDataSource@d1b84bda [ acquireIncrement -> 3, acquireRetryAttempts -> 1, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, contextClassLoaderSource -> caller, debugUnreturnedConnectionStackTraces -> false, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, forceSynchronousCheckins -> false, identityToken -> 1hge3hi9nk1ku80noopkx|3185ce3, idleConnectionTestPeriod -> 3000, initialPoolSize -> 0, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 300, maxIdleTimeExcessConnections -> 0, maxPoolSize -> 10, maxStatements -> 50, maxStatementsPerConnection -> 0, minPoolSize -> 0, nestedDataSource -> com.mchange.v2.c3p0.DriverManagerDataSource@9480ea7b [ description -> null, driverClass -> null, factoryClassLocation -> null, forceUseNamedDriverClass -> false, identityToken -> 1hge3hi9nk1ku80noopkx|26e664, jdbcUrl -> jdbc:postgresql://localhost:5432/postgres, properties -> {user=******, password=******} ], preferredTestQuery -> null, privilegeSpawnedThreads -> false, propertyCycle -> 0, statementCacheNumDeferredCloseThreads -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, usesTraditionalReflectiveProxies -> false; userOverrides: {} ], dataSourceName -> null, extensions -> {}, factoryClassLocation -> null, identityToken -> 1hge3hi9nk1ku80noopkx|3d02a858, numHelperThreads -> 3 ]
квіт. 07, 2017 2:47:07 PM org.postgresql.Driver connect
SEVERE: Connection error: 
  org.postgresql.util.PSQLException: FATAL: password authentication failed for user "user"
  --code omitted
квіт. 07, 2017 2:47:07 PM org.postgresql.Driver connect
SEVERE: Connection error: 
  org.postgresql.util.PSQLException: FATAL: password authentication failed for user "user"
  --code omitted
  -- few attempts to connect
14:47:55,009 WARN [com.mchange.v2.resourcepool.BasicResourcePool] - Having failed to acquire a resource, com.mchange.v2.resourcepool.BasicResourcePool@4519c236 is interrupting all Threads waiting on a resource to check out. Will try again in response to new client requests.

此消息之后，它不会抛出任何异常或返回

Answer 1

你可以考虑修改

<property name="hibernate.c3p0.acquireRetryAttempts" value="1"/>

回到默认值 30（或者更高），然后尝试

<property name="hibernate.c3p0.breakAfterAcquireFailure" value="true"/>

然后，如果从数据库获取连接的完整循环尝试失败（同样，应该不止一次，否则您的应用程序将非常脆弱），您的 c3p0 DataSource 将简单地中断，并进一步尝试签出Connection 将立即失败。这样做的缺点是您将失去 c3p0 在临时网络或数据库中断后“自我修复”的能力。如果临时中断导致采集周期失败，您必须自己重建 DataSource（或重新启动应用程序）。

如果您想要两全其美，请将 hibernate.c3p0.acquireRetryAttempts 设置回 30-ish，将 hibernate.c3p0.breakAfterAcquireFailure 保留为默认值 false，但编写您自己的自定义代码来测试数据库的可用性。修改上面的测试connect()函数（需要重新组织，你只想创建一次EntityManagerFactory），可能很简单...

public void connect(ConnectionSettingsModel conSet) throws Exception {

    try( Connection con = DriverManager.getConnection( conSet.getUrl(), conSet.getUser(), conSet.getPassword() ) ) {
        /* Nothing to do here, really... */
    } catch ( Exception e ) {
        System.out.println("Trouble connecting with DBMS, please check database url, username, and password.");
        throw e;  
    }

    Map<String, String> connectionProperties = new HashMap<>();
    connectionProperties.put("javax.persistence.jdbc.url", conSet.getUrl());
    connectionProperties.put("javax.persistence.jdbc.user", conSet.getUser());
    connectionProperties.put("javax.persistence.jdbc.password", conSet.getPassword());
    connectionProperties.put("hibernate.default_schema", conSet.getSchema());


    System.out.println("Before creating EM");
    EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("postgres-connect", connectionProperties);
    EntityManager entityManager = entityManagerFactory.createEntityManager();
    System.out.println("After creating EM");
 }