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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() ~[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() ~[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 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 query = cb.createTupleQuery();

Root root = query.from(BillDO.class);

List 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 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);

}