为什么同样的代码，但是添加用户是正常的，更新用户就一直报错，一行一行检查代码，发现虽然是同样的代码，但是

添加了之后，就成功不了，报服务器错误

去掉Double.parseDouble方法，连接成功，不会报服务器500错误

關於 try-catch 的部分, catch() 中的

Exceptin e

e.printStackTrace();

SQLExceptrion throwables,
throwables.printStackTrace()

這幾個東西是什麼呢?從哪邊來的呢?

希望老師可以詳細的解釋一下這幾個異常

靜態方法的 static 有什麼特性或優勢呢? 

老師好, 經過幾堂課了但是對於這行指令還是有許多不明白的地方

InputStream is = JdbcTest2.class.getClassLoader().getResourceAsStream("jdbc.properties");

可以不可以再幫我詳細解釋一下呢?

JdbcTest2.class 是指什麼呢?為什麼會突然出現這個.class 類呢?
之後的 .getClassLoader()方法又是什麼呢?
在之後的 .getResourceAsStream()方法又是什麼呢?

mac系统中修改了hosts要重启电脑才生效，有什么命令吗

老師好, 請問開頭提到的硬編碼會造成什麼樣的問題呢?

您好

当我创建job grades这个表或者往这个表里insert数据时 运行时会出现错误信息

[Err] 1055 - Expression #1 of ORDER BY clause is not in GROUP BY clause and contains nonaggregated column 'information_schema.PROFILING.SEQ' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

是版本问题吗 该怎样禁用ONLY_FULL_GROUP_BY

怎么回事？装不上，这个也没有src目录！

怎么回事？装不上，这个也没有src目录！

为什么我的这样？浏览器还报404

[INFO] Scanning for projects...
[INFO]
[INFO] ----------------< com.itbaizhan.maven:maven-helloworld >----------------
[INFO] Building 第一个maven工程 0.0.1-SNAPSHOT
[INFO] --------------------------------[ war ]---------------------------------
[INFO]
[INFO] >>> tomcat7-maven-plugin:2.1:run (default-cli) > process-classes @ maven-helloworld >>>
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ maven-helloworld ---
[WARNING] Using platform encoding (GBK actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] Copying 0 resource
[INFO]
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ maven-helloworld ---
[INFO] Nothing to compile - all classes are up to date
[INFO]
[INFO] <<< tomcat7-maven-plugin:2.1:run (default-cli) < process-classes @ maven-helloworld <<<
[INFO]
[INFO]
[INFO] --- tomcat7-maven-plugin:2.1:run (default-cli) @ maven-helloworld ---
[INFO] Running war on http://localhost:8080/
[INFO] Creating Tomcat server configuration at D:\1JAVA培训班\源码文件\maven源码(1)\maven-helloworld\target\tomcat
[INFO] create webapp with contextPath:
六月 10, 2023 11:41:07 下午 org.apache.coyote.AbstractProtocol init
信息: Initializing ProtocolHandler ["http-bio-8080"]
六月 10, 2023 11:41:07 下午 org.apache.coyote.AbstractProtocol init
严重: Failed to initialize end point associated with ProtocolHandler ["http-bio-8080"]
java.net.BindException: Address already in use: JVM_Bind <null>:8080
        at org.apache.tomcat.util.net.JIoEndpoint.bind(JIoEndpoint.java:406)
        at org.apache.tomcat.util.net.AbstractEndpoint.init(AbstractEndpoint.java:610)
        at org.apache.coyote.AbstractProtocol.init(AbstractProtocol.java:429)
        at org.apache.coyote.http11.AbstractHttp11JsseProtocol.init(AbstractHttp11JsseProtocol.java:119)
        at org.apache.catalina.connector.Connector.initInternal(Connector.java:981)
        at org.apache.catalina.util.LifecycleBase.init(LifecycleBase.java:102)
        at org.apache.catalina.core.StandardService.initInternal(StandardService.java:559)
        at org.apache.catalina.util.LifecycleBase.init(LifecycleBase.java:102)
        at org.apache.catalina.core.StandardServer.initInternal(StandardServer.java:814)
        at org.apache.catalina.util.LifecycleBase.init(LifecycleBase.java:102)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:139)
        at org.apache.catalina.startup.Tomcat.start(Tomcat.java:335)
        at org.apache.tomcat.maven.plugin.tomcat7.run.AbstractRunMojo.startContainer(AbstractRunMojo.java:1091)
        at org.apache.tomcat.maven.plugin.tomcat7.run.AbstractRunMojo.execute(AbstractRunMojo.java:512)
        at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:210)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:156)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:148)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81)
        at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:56)
        at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:305)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:192)
        at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:105)
        at org.apache.maven.cli.MavenCli.execute(MavenCli.java:972)
        at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:293)
        at org.apache.maven.cli.MavenCli.main(MavenCli.java:196)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:282)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:225)
        at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:406)
        at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:347)
Caused by: java.net.BindException: Address already in use: JVM_Bind
        at java.net.DualStackPlainSocketImpl.bind0(Native Method)
        at java.net.DualStackPlainSocketImpl.socketBind(DualStackPlainSocketImpl.java:106)
        at java.net.AbstractPlainSocketImpl.bind(AbstractPlainSocketImpl.java:387)
        at java.net.PlainSocketImpl.bind(PlainSocketImpl.java:190)
        at java.net.ServerSocket.bind(ServerSocket.java:375)
        at java.net.ServerSocket.<init>(ServerSocket.java:237)
        at java.net.ServerSocket.<init>(ServerSocket.java:181)
        at org.apache.tomcat.util.net.DefaultServerSocketFactory.createSocket(DefaultServerSocketFactory.java:49)
        at org.apache.tomcat.util.net.JIoEndpoint.bind(JIoEndpoint.java:393)
        ... 35 more

六月 10, 2023 11:41:07 下午 org.apache.catalina.core.StandardService initInternal
严重: Failed to initialize connector [Connector[HTTP/1.1-8080]]
org.apache.catalina.LifecycleException: Failed to initialize component [Connector[HTTP/1.1-8080]]
        at org.apache.catalina.util.LifecycleBase.init(LifecycleBase.java:106)
        at org.apache.catalina.core.StandardService.initInternal(StandardService.java:559)
        at org.apache.catalina.util.LifecycleBase.init(LifecycleBase.java:102)
        at org.apache.catalina.core.StandardServer.initInternal(StandardServer.java:814)
        at org.apache.catalina.util.LifecycleBase.init(LifecycleBase.java:102)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:139)
        at org.apache.catalina.startup.Tomcat.start(Tomcat.java:335)
        at org.apache.tomcat.maven.plugin.tomcat7.run.AbstractRunMojo.startContainer(AbstractRunMojo.java:1091)
        at org.apache.tomcat.maven.plugin.tomcat7.run.AbstractRunMojo.execute(AbstractRunMojo.java:512)
        at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:210)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:156)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:148)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81)
        at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:56)
        at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:305)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:192)
        at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:105)
        at org.apache.maven.cli.MavenCli.execute(MavenCli.java:972)
        at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:293)
        at org.apache.maven.cli.MavenCli.main(MavenCli.java:196)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:282)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:225)
        at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:406)
        at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:347)
Caused by: org.apache.catalina.LifecycleException: Protocol handler initialization failed
        at org.apache.catalina.connector.Connector.initInternal(Connector.java:983)
        at org.apache.catalina.util.LifecycleBase.init(LifecycleBase.java:102)
        ... 30 more
Caused by: java.net.BindException: Address already in use: JVM_Bind <null>:8080
        at org.apache.tomcat.util.net.JIoEndpoint.bind(JIoEndpoint.java:406)
        at org.apache.tomcat.util.net.AbstractEndpoint.init(AbstractEndpoint.java:610)
        at org.apache.coyote.AbstractProtocol.init(AbstractProtocol.java:429)
        at org.apache.coyote.http11.AbstractHttp11JsseProtocol.init(AbstractHttp11JsseProtocol.java:119)
        at org.apache.catalina.connector.Connector.initInternal(Connector.java:981)
        ... 31 more
Caused by: java.net.BindException: Address already in use: JVM_Bind
        at java.net.DualStackPlainSocketImpl.bind0(Native Method)
        at java.net.DualStackPlainSocketImpl.socketBind(DualStackPlainSocketImpl.java:106)
        at java.net.AbstractPlainSocketImpl.bind(AbstractPlainSocketImpl.java:387)
        at java.net.PlainSocketImpl.bind(PlainSocketImpl.java:190)
        at java.net.ServerSocket.bind(ServerSocket.java:375)
        at java.net.ServerSocket.<init>(ServerSocket.java:237)
        at java.net.ServerSocket.<init>(ServerSocket.java:181)
        at org.apache.tomcat.util.net.DefaultServerSocketFactory.createSocket(DefaultServerSocketFactory.java:49)
        at org.apache.tomcat.util.net.JIoEndpoint.bind(JIoEndpoint.java:393)
        ... 35 more

六月 10, 2023 11:41:07 下午 org.apache.catalina.core.StandardService startInternal
信息: Starting service Tomcat
六月 10, 2023 11:41:07 下午 org.apache.catalina.core.StandardEngine startInternal
信息: Starting Servlet Engine: Apache Tomcat/7.0.37

老师我找到错哪里了

这是什么问题

示例二 

我这样写是不是也可以 思路是当佣金为空时我直接就计算salary*12了 如果不为空就计算salary*12+佣金比例

select LAST_NAME,SALARY,  if(COMMISSION_PCT is NULL,SALARY*12,SALARY*12+SALARY*12*COMMISSION_PCT) from employees;

我这个也是js的问题，服务器500错误

这是为什么啊