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前言

在一次系统迭代后用户投诉说无法成功登陆系统，经过测试重现和日志定位，最后发现是由于用户在ui上进行了某些操作后，触发了堆栈溢出异常，导致数据库里的用户登陆信息表的数据被锁住，无法释放。这个表里存放的是用户的session信息。

虽然后来解决了问题，但是数据库里这个用户登录信息表里被lock住的数据始终无法释放，这导致用户永远无法登陆成功，需要手动跑SQL把锁去掉才行。

杀掉指定进程

PostgreSQL提供了两个函数：pg_cancel_backend()和pg_terminate_backend()，这两个函数的输入参数是进程PID，假定现在要杀死进程PID为20407的进程，使用方法如下：

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select pg_cancel_backend(20407);

--或者执行这个函数也可以：
select pg_terminate_backend(20407);

这两个函数区别如下：

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本文共计 : 305 字 | 阅文耗时 ≈ 1 min

!=会过滤值为null的数据

在测试数据时忽然发现，使用如下的SQL是无法查询到对应column为null的数据的：

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select * from test where name != 'Lewis';

本意是想把表里name的值不为Lewis的所有数据都搜索出来，结果发现这样写无法把name的值为null的数据也包括进来。

上面的!=换成<>也是一样的结果，这可能是因为在数据库里null是一个特殊值，有自己的判断标准，如果想要把null的数据也一起搜索出来，需要额外加上条件，如下：

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select * from test where name != 'Lewis' or name is null;

虽然这只是个小知识点，不过还是值得记录注意下，以免日后在开发中犯小错误。

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本文共计 : 2,076 字 | 阅文耗时 ≈ 10 min

问题与分析

某日忽然发现在用postman测试数据时报错如下：

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com.fasterxml.jackson.databind.JsonMappingException: could not initialize proxy [com.cbxsoftware.cbx.attachment.entity.RefAttachment#c109ec36e60c4a89a10eabc72416d984] - no Session (through reference chain: com.cbxsoftware.cbx.sampletracker.elasticsearch.entity.SampleTrackerDetailEstc["sampleTracker"]->com.cbxsoftware.cbx.sampletracker.elasticsearch.entity.SampleTrackerEstc["sampleTracker"]->com.cbxsoftware.cbx.sampletracker.entity.SampleTracker["item"]->com.cbxsoftware.cbx.item.entity.RefItem["image"]->com.cbxsoftware.cbx.image.entity.RefImage["propFormat"]->com.cbxsoftware.cbx.attachment.entity.RefAttachment$HibernateProxy$uNA5RwMT["revision"])
    at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:394)
    at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:353)
    at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:316)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:727)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
    at com.fasterxml.jackson.databind.ser.impl.UnwrappingBeanSerializer.serialize(UnwrappingBeanSerializer.java:120)
    at com.fasterxml.jackson.databind.ser.impl.UnwrappingBeanPropertyWriter.serializeAsField(UnwrappingBeanPropertyWriter.java:127)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
    at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3905)
    at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3219)
    ...
Caused by: org.hibernate.LazyInitializationException: could not initialize proxy [com.cbxsoftware.cbx.attachment.entity.RefAttachment#c109ec36e60c4a89a10eabc72416d984] - no Session
    at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:169)
    at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:309)
    at org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor.intercept(ByteBuddyInterceptor.java:45)
    at org.hibernate.proxy.ProxyConfiguration$InterceptorDispatcher.intercept(ProxyConfiguration.java:95)
    at com.cbxsoftware.cbx.attachment.entity.RefAttachment$HibernateProxy$uNA5RwMT.getRevision(Unknown Source)
    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:497)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:688)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
    ... 128 common frames omitted

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本文共计 : 3,568 字 | 阅文耗时 ≈ 15 min

问题与分析

某天领导report了一个问题：线上的CPU自从上一个版本迭代后就一直处于居高不下的状况，领导看着这段时间的曲线图判断是有两条线程在不停的死循环。

接到任务后去查看了AWS的CloudWatch，发现线上CPU确实一直居高不下，使用率基本是之前的两倍；另外发现线程使用率以比之前频繁很多。后来公司的大佬拿到dump后经过分析发现，是由正则表达式造成的CPU持续高使用率的问题。

堆栈信息如下：

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at java.util.regex.Pattern$Loop.match(Pattern.java:4787)
    at java.util.regex.Pattern$GroupTail.match(Pattern.java:4719)
    at java.util.regex.Pattern$Curly.match0(Pattern.java:4281)
    at java.util.regex.Pattern$Curly.match(Pattern.java:4236)
    at java.util.regex.Pattern$Curly.match0(Pattern.java:4274)
    at java.util.regex.Pattern$Curly.match(Pattern.java:4236)
    at java.util.regex.Pattern$GroupCurly.match0(Pattern.java:4487)
    at java.util.regex.Pattern$GroupCurly.match(Pattern.java:4407)
    at java.util.regex.Pattern$Curly.match0(Pattern.java:4274)
    at java.util.regex.Pattern$Curly.match(Pattern.java:4236)
    at java.util.regex.Pattern$GroupHead.match(Pattern.java:4660)
    at java.util.regex.Pattern$Loop.match(Pattern.java:4787)
    at java.util.regex.Pattern$GroupTail.match(Pattern.java:4719)
    at java.util.regex.Pattern$Curly.match0(Pattern.java:4281)
    at java.util.regex.Pattern$Curly.match(Pattern.java:4236)
    at java.util.regex.Pattern$Curly.match0(Pattern.java:4274)
    at java.util.regex.Pattern$Curly.match(Pattern.java:4236)
    at java.util.regex.Pattern$GroupCurly.match0(Pattern.java:4487)
    at java.util.regex.Pattern$GroupCurly.match(Pattern.java:4407)
    at java.util.regex.Pattern$Curly.match0(Pattern.java:4274)
    at java.util.regex.Pattern$Curly.match(Pattern.java:4236)
    at java.util.regex.Pattern$GroupHead.match(Pattern.java:4660)
    at java.util.regex.Pattern$Loop.match(Pattern.java:4787)
    at java.util.regex.Pattern$GroupTail.match(Pattern.java:4719)
    at java.util.regex.Pattern$Curly.match0(Pattern.java:4274)
    at java.util.regex.Pattern$Curly.match(Pattern.java:4236)
    at java.util.regex.Pattern$Curly.match0(Pattern.java:4274)
    at java.util.regex.Pattern$Curly.match(Pattern.java:4236)
    at java.util.regex.Pattern$GroupCurly.match0(Pattern.java:4487)
    at java.util.regex.Pattern$GroupCurly.match(Pattern.java:4407)
    at java.util.regex.Pattern$Curly.match0(Pattern.java:4274)
    at java.util.regex.Pattern$Curly.match(Pattern.java:4236)
    at java.util.regex.Pattern$GroupHead.match(Pattern.java:4660)
    at java.util.regex.Pattern$Loop.matchInit(Pattern.java:4803)
    at java.util.regex.Pattern$Prolog.match(Pattern.java:4743)
    at java.util.regex.Pattern$GroupCurly.match0(Pattern.java:4487)
    at java.util.regex.Pattern$GroupCurly.match(Pattern.java:4407)
    at java.util.regex.Pattern$GroupTail.match(Pattern.java:4719)
    at java.util.regex.Pattern$BranchConn.match(Pattern.java:4570)
    at java.util.regex.Pattern$BmpCharProperty.match(Pattern.java:3800)
    at java.util.regex.Pattern$Branch.match(Pattern.java:4606)
    at java.util.regex.Pattern$GroupHead.match(Pattern.java:4660)
    at java.util.regex.Pattern$Start.match(Pattern.java:3463)
    at java.util.regex.Matcher.search(Matcher.java:1248)
    at java.util.regex.Matcher.find(Matcher.java:637)
    at com.core.cbx.mybatis.plugin.sql.TableAliasParseInjector.getTableAlias(TableAliasParseInjector.java:48)

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本文共计 : 728 字 | 阅文耗时 ≈ 3 min

问题与分析

在启动公司项目时发现报错如下：

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[jetty] 2019-10-07 10:28:28.760:WARN:org.apache.jasper.compiler.Compiler:Error compiling file: D:\lewis.liu\CBX_KME\Program\jetty-temp\main\jsp\org\apache\jsp\invalidatePrevSession_jsp.javanull
[jetty] 2019-10-07 10:28:28.763:WARN::/main/invalidatePrevSession.jsp
[jetty] org.apache.jasper.JasperException: PWC6033: Unable to compile class for JSP||PWC6199: Generated servlet error:|The import com.core.cbx.ui.zk.cul.grid.renderer.FileLinkCellRenderer cannot be resolved||
[jetty]     at org.apache.jasper.compiler.DefaultErrorHandler.javacError(DefaultErrorHandler.java:123)
[jetty]     at org.apache.jasper.compiler.ErrorDispatcher.javacError(ErrorDispatcher.java:296)
[jetty]     at org.apache.jasper.compiler.Compiler.generateClass(Compiler.java:376)
[jetty]     at org.apache.jasper.compiler.Compiler.compile(Compiler.java:437)
[jetty]     at org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:608)
[jetty]     at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:360)
[jetty]     at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:486)
[jetty]     at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:380)
[jetty]     at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
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