spring boot JPA 数据库连接池释放

当JPA获取数据库数据连接时，如果连接数超过最大连接数的配置，系统就会报错：

Unable to acquire JDBC Connection

和：

Caused by: java.sql.SQLTransientConnectionException: HikariPool-1 - Connection is not available, request timed out after 30002ms.
    at com.zaxxer.hikari.pool.HikariPool.createTimeoutException(HikariPool.java:676) ~[HikariCP-3.2.0.jar:?]
    at com.zaxxer.hikari.pool.HikariPool.getConnection(HikariPool.java:190) ~[HikariCP-3.2.0.jar:?]
    at com.zaxxer.hikari.pool.HikariPool.getConnection(HikariPool.java:155) ~[HikariCP-3.2.0.jar:?]
    at com.zaxxer.hikari.HikariDataSource.getConnection(HikariDataSource.java:128) ~[HikariCP-3.2.0.jar:?]
    at com.zaxxer.hikari.HikariDataSource$$FastClassBySpringCGLIB$$eeb1ae86.invoke(<generated>) ~[HikariCP-3.2.0.jar:?]
    at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218) ~[spring-core-5.1.8.RELEASE.jar:5.1.8.RELEASE]
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:749) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
    at org.springframework.aop.support.DelegatingIntroductionInterceptor.doProceed(DelegatingIntroductionInterceptor.java:136) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
    at org.springframework.aop.support.DelegatingIntroductionInterceptor.invoke(DelegatingIntroductionInterceptor.java:124) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
    at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:688) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
    at com.zaxxer.hikari.HikariDataSource$$EnhancerBySpringCGLIB$$5e528ef9.getConnection(<generated>) ~[HikariCP-3.2.0.jar:?]
    at org.hibernate.engine.jdbc.connections.internal.DatasourceConnectionProviderImpl.getConnection(DatasourceConnectionProviderImpl.java:122) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.internal.NonContextualJdbcConnectionAccess.obtainConnection(NonContextualJdbcConnectionAccess.java:35) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl.acquireConnectionIfNeeded(LogicalConnectionManagedImpl.java:106) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl.getPhysicalConnection(LogicalConnectionManagedImpl.java:136) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl.connection(StatementPreparerImpl.java:47) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$5.doPrepare(StatementPreparerImpl.java:146) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$StatementPreparationTemplate.prepareStatement(StatementPreparerImpl.java:172) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl.prepareQueryStatement(StatementPreparerImpl.java:148) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.Loader.prepareQueryStatement(Loader.java:1984) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1914) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1892) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.Loader.doQuery(Loader.java:937) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:340) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.Loader.doList(Loader.java:2689) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.Loader.doList(Loader.java:2672) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2506) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.Loader.list(Loader.java:2501) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.loader.hql.QueryLoader.list(QueryLoader.java:504) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.list(QueryTranslatorImpl.java:395) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.engine.query.spi.HQLQueryPlan.performList(HQLQueryPlan.java:220) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1508) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.query.internal.AbstractProducedQuery.doList(AbstractProducedQuery.java:1537) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.query.internal.AbstractProducedQuery.list(AbstractProducedQuery.java:1505) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    ... 64 more

连接数的配置：

spring:
  datasource:
    hikari:
      minimum-idle: 1
      maximum-pool-size: 5

如果不配置的话，默认都是10.

我们使用entitymanager进行查询和其他操作时，调用这个方法org.springframework.orm.jpa.SharedEntityManagerCreator.DeferredQueryInvocationHandler#invoke，红色代码是会数据库连接池进行进行释放。SharedEntityManagerCreator这个类是这个关键，有兴趣可以重点看一下。

@Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            // Invocation on Query interface coming in...

            if (method.getName().equals("equals")) {
                // Only consider equal when proxies are identical.
                return (proxy == args[0]);
            }
            else if (method.getName().equals("hashCode")) {
                // Use hashCode of EntityManager proxy.
                return hashCode();
            }
            else if (method.getName().equals("unwrap")) {
                // Handle JPA 2.0 unwrap method - could be a proxy match.
                Class<?> targetClass = (Class<?>) args[0];
                if (targetClass == null) {
                    return this.target;
                }
                else if (targetClass.isInstance(proxy)) {
                    return proxy;
                }
            }
            else if (method.getName().equals("getOutputParameterValue")) {
                if (this.entityManager == null) {
                    Object key = args[0];
                    if (this.outputParameters == null || !this.outputParameters.containsKey(key)) {
                        throw new IllegalArgumentException("OUT/INOUT parameter not available: " + key);
                    }
                    Object value = this.outputParameters.get(key);
                    if (value instanceof IllegalArgumentException) {
                        throw (IllegalArgumentException) value;
                    }
                    return value;
                }
            }

            // Invoke method on actual Query object.
            try {
                Object retVal = method.invoke(this.target, args);
                if (method.getName().equals("registerStoredProcedureParameter") && args.length == 3 &&
                        (args[2] == ParameterMode.OUT || args[2] == ParameterMode.INOUT)) {
                    if (this.outputParameters == null) {
                        this.outputParameters = new LinkedHashMap<>();
                    }
                    this.outputParameters.put(args[0], null);
                }
                return (retVal == this.target ? proxy : retVal);
            }
            catch (InvocationTargetException ex) {
                throw ex.getTargetException();
            }
            finally {
                if (queryTerminatingMethods.contains(method.getName())) {
                    // Actual execution of the query: close the EntityManager right
                    // afterwards, since that was the only reason we kept it open.
                    if (this.outputParameters != null && this.target instanceof StoredProcedureQuery) {
                        StoredProcedureQuery storedProc = (StoredProcedureQuery) this.target;
                        for (Map.Entry<Object, Object> entry : this.outputParameters.entrySet()) {
                            try {
                                Object key = entry.getKey();
                                if (key instanceof Integer) {
                                    entry.setValue(storedProc.getOutputParameterValue((Integer) key));
                                }
                                else {
                                    entry.setValue(storedProc.getOutputParameterValue(key.toString()));
                                }
                            }
                            catch (IllegalArgumentException ex) {
                                entry.setValue(ex);
                            }
                        }
                    }
                    EntityManagerFactoryUtils.closeEntityManager(this.entityManager);
                    this.entityManager = null;
                }
            }
        }

如果我们使用@Transactional注解时，就不是上会的代码来释放连接池的，是下面的代码：

org.springframework.orm.jpa.JpaTransactionManager#doCleanupAfterCompletion

@Override
    protected void doCleanupAfterCompletion(Object transaction) {
        JpaTransactionObject txObject = (JpaTransactionObject) transaction;

        // Remove the entity manager holder from the thread, if still there.
        // (Could have been removed by EntityManagerFactoryUtils in order
        // to replace it with an unsynchronized EntityManager).
        if (txObject.isNewEntityManagerHolder()) {
            TransactionSynchronizationManager.unbindResourceIfPossible(obtainEntityManagerFactory());
        }
        txObject.getEntityManagerHolder().clear();

        // Remove the JDBC connection holder from the thread, if exposed.
        if (getDataSource() != null && txObject.hasConnectionHolder()) {
            TransactionSynchronizationManager.unbindResource(getDataSource());
            ConnectionHandle conHandle = txObject.getConnectionHolder().getConnectionHandle();
            if (conHandle != null) {
                try {
                    getJpaDialect().releaseJdbcConnection(conHandle,
                            txObject.getEntityManagerHolder().getEntityManager());
                }
                catch (Exception ex) {
                    // Just log it, to keep a transaction-related exception.
                    logger.error("Could not close JDBC connection after transaction", ex);
                }
            }
        }

        getJpaDialect().cleanupTransaction(txObject.getTransactionData());

        // Remove the entity manager holder from the thread.
        if (txObject.isNewEntityManagerHolder()) {
            EntityManager em = txObject.getEntityManagerHolder().getEntityManager();
            if (logger.isDebugEnabled()) {
                logger.debug("Closing JPA EntityManager [" + em + "] after transaction");
            }
            EntityManagerFactoryUtils.closeEntityManager(em);
        }
        else {
            logger.debug("Not closing pre-bound JPA EntityManager after transaction");
        }
    }

这是两种逻辑，如果我们在@Transactional注解的方法会使用em的方法进行数据库的操作的话，会采用第二种方式来释放，

如果不加@Transactional注解的话，采用第一种方法来释放。

所以为了避免发现特殊情况，出现不释放连接池的情况，可以采用加@Transactional注解的方式 处理，如下面的代码：

@Transactional
    @RequestMapping("/search")
    public List search() {

        CriteriaBuilder cb = entityManager.getCriteriaBuilder();
        CriteriaQuery<Tuple> query = cb.createTupleQuery();
        Root<BillDO> root = query.from(BillDO.class);

        List<Predicate> predicates = new ArrayList<>();
        predicates.add(cb.equal(root.get(BillDO.Fields.hisId),"3035-M6748091-1695675075-1"));
        query.where(Iterables.toArray(predicates, Predicate.class));

        query.multiselect(Arrays.asList(
                root.get(BillDO.Fields.hisId),
                root.get(BillDO.Fields.hospitalName),
                root.get(BillDO.Fields.hospitalId),
                root.get(BillDO.Fields.id),
                root.get(BillDO.Fields.diseaseName)
        ));
        TypedQuery<Tuple> typedQuery = entityManager.createQuery(query);
       // Query unwrap = typedQuery.unwrap(Query.class);
        QueryImpl unwrap = typedQuery.unwrap(QueryImpl.class);

        Query query1 = unwrap.setResultTransformer(
                new CisAliasToBeanResultTransformer(BillDO.class, query.getSelection()));

        List resultList = query1.list();
        return resultList;
    }

 如果不加@Transactional注解的话，那么连接池将不会释放，连接数会一直增加，直到超过最大连接数的配置。

可以通过下面的配置，来观察hikari数据库连接连接池的变化：

logging:
  level:
    root: warn
    com:
      zaxxer:
        hikari: trace

这个代码是Hikari回收连接的：com.zaxxer.hikari.pool.HikariPool#recycle

  @Override
   void recycle(final PoolEntry poolEntry)
   {
      metricsTracker.recordConnectionUsage(poolEntry);

      connectionBag.requite(poolEntry);
   }