我刚开始使用springboot。最近出现了如下错误。与其他stackoverflow帖子也检查过了..似乎什么都不起作用..我正在使用intellij idea，最近遇到了这个问题..有什么帮助吗？
如post:- severe：无法使用context.xml文件创建池-tomcat 7的初始连接

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Could not create connection to database server.
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:na]
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:488) ~[na:na]
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.Util.getInstance(Util.java:408) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:918) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:897) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:886) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:860) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2330) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2083) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:806) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:na]
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:488) ~[na:na]
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:410) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:328) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at org.apache.tomcat.jdbc.pool.PooledConnection.connectUsingDriver(PooledConnection.java:310) ~[tomcat-jdbc-8.5.6.jar:na]
    at org.apache.tomcat.jdbc.pool.PooledConnection.connect(PooledConnection.java:203) ~[tomcat-jdbc-8.5.6.jar:na]
    at org.apache.tomcat.jdbc.pool.ConnectionPool.createConnection(ConnectionPool.java:718) [tomcat-jdbc-8.5.6.jar:na]
    at org.apache.tomcat.jdbc.pool.ConnectionPool.borrowConnection(ConnectionPool.java:650) [tomcat-jdbc-8.5.6.jar:na]
    at org.apache.tomcat.jdbc.pool.ConnectionPool.init(ConnectionPool.java:468) [tomcat-jdbc-8.5.6.jar:na]
    at org.apache.tomcat.jdbc.pool.ConnectionPool.<init>(ConnectionPool.java:143) [tomcat-jdbc-8.5.6.jar:na]
    at org.apache.tomcat.jdbc.pool.DataSourceProxy.pCreatePool(DataSourceProxy.java:118) [tomcat-jdbc-8.5.6.jar:na]
    at org.apache.tomcat.jdbc.pool.DataSourceProxy.createPool(DataSourceProxy.java:107) [tomcat-jdbc-8.5.6.jar:na]
    at org.apache.tomcat.jdbc.pool.DataSourceProxy.getConnection(DataSourceProxy.java:131) [tomcat-jdbc-8.5.6.jar:na]
    at org.hibernate.engine.jdbc.connections.internal.DatasourceConnectionProviderImpl.getConnection(DatasourceConnectionProviderImpl.java:122) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess.obtainConnection(JdbcEnvironmentInitiator.java:180) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.initiateService(JdbcEnvironmentInitiator.java:68) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.initiateService(JdbcEnvironmentInitiator.java:35) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.boot.registry.internal.StandardServiceRegistryImpl.initiateService(StandardServiceRegistryImpl.java:88) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.createService(AbstractServiceRegistryImpl.java:254) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:228) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:207) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.engine.jdbc.internal.JdbcServicesImpl.configure(JdbcServicesImpl.java:51) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.boot.registry.internal.StandardServiceRegistryImpl.configureService(StandardServiceRegistryImpl.java:94) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:237) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:207) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.handleTypes(MetadataBuildingProcess.java:352) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:111) [hibernate-core-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.metadata(EntityManagerFactoryBuilderImpl.java:847) [hibernate-entitymanager-5.0.11.Final.jar:5.0.11.Final]
    at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:874) [hibernate-entitymanager-5.0.11.Final.jar:5.0.11.Final]
    at org.springframework.orm.jpa.vendor.SpringHibernateJpaPersistenceProvider.createContainerEntityManagerFactory(SpringHibernateJpaPersistenceProvider.java:60) [spring-orm-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:353) [spring-orm-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.buildNativeEntityManagerFactory(AbstractEntityManagerFactoryBean.java:373) [spring-orm-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:362) [spring-orm-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1648) [spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1585) [spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:554) [spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483) [spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306) [spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) [spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) [spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197) [spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1081) [spring-context-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:856) [spring-context-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542) [spring-context-4.3.5.RELEASE.jar:4.3.5.RELEASE]
    at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:122) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:761) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:371) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1186) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1175) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
    at com.bookstore.BookstoreApplication.main(BookstoreApplication.java:24) [classes/:na]
Caused by: java.lang.NullPointerException: null
    at com.mysql.jdbc.ConnectionImpl.getServerCharset(ConnectionImpl.java:2997) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.MysqlIO.sendConnectionAttributes(MysqlIO.java:1934) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1863) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1226) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2253) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2284) ~[mysql-connector-java-5.1.40.jar:5.1.40]
    ... 57 common frames omitted

pom.xml文件

<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-thymeleaf</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-jdbc</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-security</artifactId>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>

        <dependency>
            <groupId>org.thymeleaf.extras</groupId>
            <artifactId>thymeleaf-extras-springsecurity4</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context-support</artifactId>
        </dependency>

        <dependency>
            <groupId>javax.mail</groupId>
            <artifactId>mail</artifactId>
            <version>1.4.7</version>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>