java 随机生成验证码 思路 通过ASCII表生成大小写验证码 public static void main(String[] args) { System.out.println( createCode(6)); } public static String createCode(int n){ //int n 表示随机生成几位 Random r = new Random(); String code = ""; for (int i = 0; i < n; i++) { int type = r.nextInt(3); switch (type){ case 0: //大写字母 ASCII码 char ch = (char)(r.nextInt(25)+65); code += ch; break; case 1: //小写字母 ASCII码 char ch1 = (char)(r.nextInt(25)+97); code += ch1; break; case 2: //数字 code += r.nextInt(10); break; } } return code ; } }

tomcat 报错： A child container failed during start 解决方法 错误详情涓ラ��: A child container failed during start java.util.concurrent.ExecutionException: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Tomcat].StandardHost[localhost].StandardContext[]] at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122) at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191) at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1123) at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:800) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549) at java.base/java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:264) at java.base/java.util.concurrent.FutureTask.run(FutureTask.java) at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) at java.base/java.lang.Thread.run(Thread.java:834) Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Tomcat].StandardHost[localhost].StandardContext[]] at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:154) ... 7 more Caused by: java.lang.LinkageError: loader constraint violation: loader org.apache.catalina.loader.WebappClassLoader @53e582eb (instance of org.apache.catalina.loader.WebappClassLoader, child of org.codehaus.plexus.classworlds.realm.ClassRealm @257e0827 org.codehaus.plexus.classworlds.realm.ClassRealm) wants to load interface javax.servlet.ServletContext. A different interface with the same name was previously loaded by org.codehaus.plexus.classworlds.realm.ClassRealm @257e0827 (instance of org.codehaus.plexus.classworlds.realm.ClassRealm, child of 'bootstrap'). at java.base/java.lang.ClassLoader.defineClass1(Native Method) at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1016) at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:174) at java.base/java.net.URLClassLoader.defineClass(URLClassLoader.java:550) at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:458) at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:452) at java.base/java.security.AccessController.doPrivileged(Native Method) at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:451) at org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:1191) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1669) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1547) at org.springframework.web.SpringServletContainerInitializer.onStartup(SpringServletContainerInitializer.java:165) at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5423) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) ... 7 more 12�� 19, 2019 3:20:08 涓��� org.apache.catalina.core.ContainerBase startInternal 涓ラ��: A child container failed during start java.util.concurrent.ExecutionException: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Tomcat].StandardHost[localhost]] at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122) at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191) at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1123) at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:302) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) at org.apache.catalina.core.StandardService.startInternal(StandardService.java:443) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:732) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) at org.apache.catalina.startup.Tomcat.start(Tomcat.java:341) at org.apache.tomcat.maven.plugin.tomcat7.run.AbstractRunMojo.startContainer(AbstractRunMojo.java:1238) at org.apache.tomcat.maven.plugin.tomcat7.run.AbstractRunMojo.execute(AbstractRunMojo.java:592) at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134) at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208) at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:154) at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:146) 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:51) at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128) at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:309) at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:194) at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:107) at org.apache.maven.cli.MavenCli.execute(MavenCli.java:955) at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:290) at org.apache.maven.cli.MavenCli.main(MavenCli.java:194) at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.base/java.lang.reflect.Method.invoke(Method.java:566) at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289) at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229) at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415) at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356) at org.codehaus.classworlds.Launcher.main(Launcher.java:47) Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Tomcat].StandardHost[localhost]] at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:154) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549) at java.base/java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:264) at java.base/java.util.concurrent.FutureTask.run(FutureTask.java) at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) at java.base/java.lang.Thread.run(Thread.java:834) Caused by: org.apache.catalina.LifecycleException: A child container failed during start at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1131) at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:800) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) ... 7 more 解决方法pom.xml文件中servlet-api和jsp-api的坐标中缺少scope，加载<scrop>就可以 <!--servlet和jsp--> <dependency> <groupId>javax.servlet</groupId> <artifactId>servlet-api</artifactId> <version>2.5</version> <scope>provided</scope> </dependency> <dependency> <groupId>javax.servlet.jsp</groupId> <artifactId>jsp-api</artifactId> <version>2.0</version> <scope>provided</scope> </dependency>

解决idea maven配置了Tomcat却在插件窗口不显示的问题 **注意看plugins标签所在的位置plugin的父标签plugins 可以直接放在build标签下也可以放在pluginManagement标签下而我们在创建maven项目的时候 默认存在的那个plugins标签是在pluginManagement标签下的而pluginManagement标签只是对插件的一种声明 而不会对插件进行加载所以解决办法如下：我们要在build标签下再写一个plugins标签然后再把plugin标签放进去下面是完整的pom.xml文件**<?xml version="1.0" encoding="UTF-8"?> <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>org.example</groupId> <artifactId>hello_web2</artifactId> <version>1.0-SNAPSHOT</version> <packaging>war</packaging> <name>hello_web2 Maven Webapp</name> <!-- FIXME change it to the project's website --> <url>http://www.example.com</url> <properties> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> <maven.compiler.source>1.8</maven.compiler.source> <maven.compiler.target>1.8</maven.compiler.target> </properties> <dependencies> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>4.11</version> <scope>test</scope> </dependency> </dependencies> <build> <finalName>hello_web2</finalName> <plugins> <plugin> <groupId>org.apache.tomcat.maven</groupId> <artifactId>tomcat7-maven-plugin</artifactId> <version>2.2</version> <configuration> <!--配置端口号--> <port>80</port> <!--配置项目路径--> <path>/</path> </configuration> </plugin> </plugins> <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) --> <plugins> <plugin> <artifactId>maven-clean-plugin</artifactId> <version>3.1.0</version> </plugin> <!-- see http://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_war_packaging --> <plugin> <artifactId>maven-resources-plugin</artifactId> <version>3.0.2</version> </plugin> <plugin> <artifactId>maven-compiler-plugin</artifactId> <version>3.8.0</version> </plugin> <plugin> <artifactId>maven-surefire-plugin</artifactId> <version>2.22.1</version> </plugin> <plugin> <artifactId>maven-war-plugin</artifactId> <version>3.2.2</version> </plugin> <plugin> <artifactId>maven-install-plugin</artifactId> <version>2.5.2</version> </plugin> <plugin> <artifactId>maven-deploy-plugin</artifactId> <version>2.8.2</version> </plugin> </plugins> </pluginManagement> </build> </project>具体位置如下图所示

Java Cookie-设置/获取 判断用户是否为第一次访问 且记录访问时间 小Demo protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //获取cookies response.setContentType("text/html;charset=utf-8"); Cookie[] cookies = request.getCookies(); boolean flag = false; //返回数组遍历数组 寻找 lastTime参数 if (cookies != null && cookies.length >0){ //遍历 for (Cookie cookie : cookies) { //获取 cookie 键 寻找 lastTime String name = cookie.getName(); if ("lastTime".equals(name)){ flag = true; //如果有 就不是第一次登录 Date date = new Date(); //设置时间戳格式 SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日HH:mm:ss"); //返回日期 String src_date = sdf.format(date); //设置 cookie cookie.setValue(src_date); //设置cookie 存活时间 cookie.setMaxAge( 60 * 60 * 24 * 30 ); //添加cookie response.addCookie(cookie); //获取 cookie String value = cookie.getValue(); //打印信息 response.getWriter().write("<h1>欢迎回来，您上次的访问时间为"+ value + "</h1>"); } } } if (cookies == null || cookies.length == 0 || flag == false) { Date date = new Date(); //设置时间戳格式 SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日HH:mm:ss"); //返回日期 String src_date = sdf.format(date); //设置 cookie Cookie cookie = new Cookie("lastTime",src_date); //设置cookie 存活时间 cookie.setMaxAge( 60 * 60 * 24 * 30 ); //获取 cookie response.addCookie(cookie); response.getWriter().write("欢迎用户，您是第一次访问登录！"); } }

Java反射学习 基础案例--小记 package cc.bltang.demo01; import java.io.IOException; import java.io.InputStream; import java.lang.reflect.Constructor; import java.lang.reflect.Method; import java.util.Properties; public class ReflectDemo02 { public static void main(String[] args) throws Exception { //反射 案例 Properties pro = new Properties(); //加载配置文件，转换为一个集合 //获取Class目录下的配置文件 ClassLoader classLoader = ReflectDemo02.class.getClassLoader(); InputStream is = classLoader.getResourceAsStream("pro.properties"); pro.load(is); //获取配置文件中定义的数据 String className = pro.getProperty("className"); String methodName = pro.getProperty("methodName"); //加载类进内存 Class<?> aClass = Class.forName(className); //创建对象 // Constructor<?> c = aClass.getConstructor(); // Object obj = c.newInstance(); Object obj = aClass.newInstance(); //获取方法对象 Method method = aClass.getMethod(methodName); //执行方法 method.invoke(obj); } }

Java IO字节缓冲流实现复制文件。 -小记 package demo16; import java.io.*; public class Demo03CopyFor { public static void main(String[] args) throws IOException { long e = System.currentTimeMillis(); FileInputStream fis = new FileInputStream("D:\\C0103.MP4"); BufferedInputStream bis = new BufferedInputStream(fis); FileOutputStream fos = new FileOutputStream("E:\\C0103.MP4"); BufferedOutputStream bos = new BufferedOutputStream(fos); byte[] bytes = new byte[10240]; int len = 0; while ((len = bis.read(bytes))!= -1){ bos.write(bytes); } bis.close(); bos.close(); long s = System.currentTimeMillis(); System.out.println("系统总耗时："+(s-e)+"毫秒~"); } } 系统总耗时：40毫秒~

Java 斗地主综合案例：有序版--小记 结果：周润发:小王2♣A♦Q♦J♣10♠10♥9♥9♦8♠7♥6♦5♦5♣4♦3♥3♦刘德华:大王2♠2♥2♦A♥K♠K♥K♦K♣J♦10♣8♦6♠6♣4♠3♠3♣周星驰:A♠Q♠Q♥J♠J♥10♦9♠9♣8♥8♣7♦7♣6♥5♠5♥4♥4♣底牌:A♣Q♣7♠import java.util.ArrayList; import java.util.Collections; import java.util.HashMap; import java.util.List; /* 1.准备牌 2.洗牌 3.发牌 4.排序 5.看牌 */ public class Doudizhu { public static void main(String[] args) { //准备牌两个集合，一个存储牌一个存储牌的引索序列号。 //创建一个Map集合，存储拍的引索和组装好的牌 HashMap<Integer,String> poker = new HashMap<>(); //创建一个List 集合 存储牌的引索序列号 ArrayList<Integer> pokerIndex = new ArrayList<>(); List<String> colors = List.of("♠", "♥", "♦", "♣"); List<String> number = List.of("2", "A", "K", "Q", "J", "10", "9", "8", "7", "6", "5", "4", "3"); //首先添加大王，小王 int index = 0; poker.put(index,"大王"); pokerIndex.add(index); index++; poker.put(index,"小王"); pokerIndex.add(index); index++; System.out.println(poker); System.out.println(pokerIndex); for (String numbers : number) { for (String color : colors) { poker.put(index,numbers + color); pokerIndex.add(index); index++; } } //2.洗牌 Collections shuffle(list) 方法 就行随机置放 Collections.shuffle(pokerIndex); System.out.println(pokerIndex); //3.发牌 创建玩家 以及底牌 ArrayList<Integer> player01 = new ArrayList<>(); ArrayList<Integer> player02 = new ArrayList<>(); ArrayList<Integer> player03 = new ArrayList<>(); ArrayList<Integer> diPai = new ArrayList<>(); //发牌判断 循环发牌 for (int i = 0; i < pokerIndex.size(); i++) { //获取牌的每一个索引 Integer in = pokerIndex.get(i); //进行底牌判断 if(i >= 51){ //给底牌发牌 diPai.add(in); } else if (i % 3 ==0){ //给玩家一发牌 player01.add(in); }else if (i % 3 ==1){ //给玩家一发牌 player02.add(in); }else if (i % 3 ==2){ //给玩家一发牌 player03.add(in); } } //4.排序 利用 Collections。sort（list） 方法 给牌排序 Collections.sort(player01); Collections.sort(player02); Collections.sort(player03); Collections.sort(diPai); getLook("周润发",poker,player01); getLook("刘德华",poker,player02); getLook("周星驰",poker,player03); getLook("底牌",poker,diPai); } public static void getLook(String name,HashMap<Integer,String> poker,ArrayList<Integer> list){ System.out.print(name+ ":"); for (Integer key : list) { String value = poker.get(key); System.out.print( value +""); } System.out.println(); } }