java-cheat-sheet.md

java cheat sheet

links

• java 8 download
• java decompiler, class decompile
• useful links
• source code examples
• source code examples
• java 8 json
• java links
• profiler
• byte code transformation
• docker container creator

heap dump, jmap

# find process id for target java process
ps auxf

# find amount of threads inside JVM
ps -eww H -p $JAVA_PROCESS_ID

# make heap dump
jmap -histo $JAVA_PROCESS_ID
jmap -clstats $JAVA_PROCESS_ID
jmap -heap $JAVA_PROCESS_ID
jmap -dump:format=b,live,file=$PATH_TO_OUTPUT_HEAP_DUMP $JAVA_PROCESS_ID

java repl java code evaluation

$JAVA_HOME/bin/jshell -v

import java.util.UUID;
UUID.randomUUID();

execute javascript code

#! /usr/bin/env jjs
print('start of the script');
var variableInScript = 5 * 3;
print(" output from script ${threeyr}");

execute script code

# inline
jrunscript -e "cat('https://google.com')"
# js repl
jrunscript

js> t = new java.lang.Thread(function() { print('java Thread in script\n'); })
js> t.start()

JVM parameters

-Xmx56000m 
-XX:+HeapDumpOnOutOfMemoryError 
-XX:HeapDumpPath==/usr/src/bin/log/ 
-XX:OnOutOfMemoryError=/usr/src/bin/start-app.sh
-XX:ErrorFile=/usr/src/bin/log/fatal-errors.log 
-XX:+PrintGCDetails 
-XX:+UseG1GC 
-Xloggc:/usr/src/bin/log/jvm-gc.log 

JMX java inspect java state java stuck

# command line argument
-Dcom.sun.management.jmxremote
-Dcom.sun.management.jmxremote.port=5006
-Dcom.sun.management.jmxremote.rmi.port=5006
-Djava.rmi.server.hostname=127.0.0.1 
-Dcom.sun.management.jmxremote.local.only=false
-Dcom.sun.management.jmxremote.authenticate=false
-Dcom.sun.management.jmxremote.ssl=false
# additional parameters
-Djmx.rmi.registry.port=5006 
-Djmx.rmi.port=5006

# OpenShift settings
# connect to openshift
$ oc login $OC_HOST:8443
 
# forward ports from localhost to pod
# oc port-forward $POD_NAME <local port>:<remote port>
$ oc port-forward $POD_NAME 5006
 
# e.g. connect to the jmx port with visual vm
visualvm --openjmx localhost:5006
jconsole localhost:5006

visualvm

visual vm download

set specific jdk

vim etc/visualvm.conf

visualvm_jdkhome="/home/my-user/.sdkman/candidates/java/18.0.1-oracle"

for connecting to jvm - select "new jmx connection"

java application debug, remote debug

java -jar ...

-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005

-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=localhost:5005

The Java image on OpenShift has built-in support for remote debugging

# environment variables
JAVA_DEBUG=true  
# or
oc set env dc/inventory JAVA_DEBUG=true

you can create SSH tunnel between your local machine and remote: ( execute next line from local machine )

ssh -L 7000:127.0.0.1:5005 $REMOTE_USER@$REMOTE_HOST

and your connection url will looks like: 127.0.0.1:7000
or
connect with local openshift client

# connect to openshift
$ oc login $OPENSHIFT_HOST:8443

# forward the jmx ports
$ oc port-forward $POD_NAME 5005

java application console debug headless debug

connect to process

jdb -attach localhost:5005
jdb -attach localhost:5005 -sourcepath :src/main/java/

commands inside debugger

# set breakpoint on line
stop at com.ubs.ad.data.interval.v2.IntervalServiceImpl:189
# set breakpoint on method
stop at com.ubs.ad.data.interval.v2.IntervalServiceImpl.getAvailability

# print list of breakpoints
clear  

# remove breakpoint 
clear com.ubs.ad.data.interval.v2.IntervalServiceImpl:189

# print local variables 
locals

# for all methods need to use full name of the class
print com.ubs.interval.IntervalValidator.isContributorIdValid(subscriber)
eval com.ubs.interval.IntervalValidator.isContributorIdValid(subscriber)

# print current stack trace, print position
where 

print intervalsIdList
dump intervalsIdList
set intervalsIdList=new ArrayList<>();

movements inside debugger

next                      -- step one line (step OVER calls)
cont                      -- continue execution from breakpoint

step                      -- execute current line ( step in )
step up                   -- execute until the current method returns to its caller
stepi                     -- execute current instruction

java agent

-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=9010

java -javaagent:<agent-path.jar>=<agent args> -jar <your-jar.jar>

attach to process, agent to pid, java agent

// Using com.sun.tools.attach.VirtualMachine:
VirtualMachine vm = VirtualMachine.attach(pid)
vm.loadAgent(jarPath, agentArgS)
vm.detach()

get current java version java runtime version

System.getProperty("java.runtime.version");

set proxy system properties

System.getProperties().put("http.proxyHost", "someProxyURL");
System.getProperties().put("http.proxyPort", "someProxyPort");

-Dhttp.proxyHost=127.0.0.1 -Dhttp.proxyPort=3128 -Dhttps.proxyHost=127.0.0.1 -Dhttps.proxyPort=3129

export _JAVA_OPTIONS="-Dhttp.proxyHost=127.0.0.1 -Dhttp.proxyPort=3128 -Dhttps.proxyHost=127.0.0.1 -Dhttps.proxyPort=3128 -Dhttp.nonProxyHosts=localhost|*.ubsgroup.net|*.zur -Dhttps.nonProxyHosts=localhost|*.ubsgroup.net|*.muc"

print all classes for running application

java -verbose:class -version
java -verbose:class --classpath:my.jar  ClassWithMain

certificates

create certificate

Browser.address url.lock(broken lock) -> Certificate -> Details -> Copy to File -> Base-64 encoded X.509 -> metrics.cer
# keytool -printcert -sslserver path.to.server.com:443

read certificate

openssl x509 -text -noout -in elastic-staging.cer 

import certificate to truststore

ls myTrustStore && rm myTrustStore;
CERT_FILE=metrics.cer
keytool -printcert -rfc -file $CERT_FILE
keytool -printcert -file $CERT_FILE

CERT_ALIAS=metrics
TRUST_STORE=myTrustStore
keytool -import -file $CERT_FILE -alias $CERT_ALIAS -keystore $TRUST_STORE

# list of certificates in truststore 
keytool -list -keystore $TRUST_STORE

# export and print certificate
keytool -exportcert -keystore $TRUST_STORE -alias $CERT_ALIAS -file out.crt
keytool -printcert -file out.crt

create empty truststore

TRUST_STORE=emptyStore
ls $TRUST_STORE && rm $TRUST_STORE;
CERT_ALIAS=garbage
STORE_PASS="my_pass"

# create truststore with one record
keytool -genkeypair -alias $CERT_ALIAS -storepass $STORE_PASS -keypass secretPassword -keystore $TRUST_STORE -dname "CN=Developer, OU=Department, O=Company, L=City, ST=State, C=CA"
# list of records
keytool -list -keystore $TRUST_STORE -storepass $STORE_PASS
## delete records
keytool -delete -alias $CERT_ALIAS -storepass $STORE_PASS -keystore $TRUST_STORE
# list of records 
keytool -list -keystore $TRUST_STORE -storepass $STORE_PASS

execute application from java, execute sub-process, start program

start and waiting for finish

new ProcessExecutor().commandSplit(executeLine).execute();

start in separate process without waiting

new ProcessExecutor().commandSplit(executeLine).start();

with specific directory

new ProcessExecutor().commandSplit(executeLine).directory(executableFile.getParentFile()).start();

@Null

use Optional.absent

locking

Reentrant lock == Semaphore

jshell

print import

/!

help

/help
/?

exit

/exit

JAR

create jar based on compiled files

jar cf WC2.jar *.class

take a look into jar, list all files into jar

jar tvf WC2.jar

print loaded classes during start application

java -verbose app

Security

collaboration with underlying system javacallbacks

LOG4j

log4j configuration key for JVM

-Dlog4j.configuration={path to file}

log4j override configuration from code

Properties props = new Properties();
props.put("log4j.rootLogger", level+", stdlog");
props.put("log4j.appender.stdlog", "org.apache.log4j.ConsoleAppender");
props.put("log4j.appender.stdlog.target", "System.out");
props.put("log4j.appender.stdlog.layout", "org.apache.log4j.PatternLayout");
props.put("log4j.appender.stdlog.layout.ConversionPattern","%d{HH:mm:ss} %-5p %-25c{1} :: %m%n");
// Execution logging
props.put("log4j.logger.com.hp.specific", level);
// Everything else 
props.put("log4j.logger.com.hp", level);
LogManager.resetConfiguration();
PropertyConfigurator.configure(props);

log4j file configuration

<?xml version="1.0" encoding="UTF-8"?>
    <Configuration status="WARN">
      <Appenders>
        <Console name="Console" target="SYSTEM_OUT">
          <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
        </Console>
      </Appenders>
      <Loggers>
        <Logger name="com.foo.Bar" level="debug">
          <AppenderRef ref="Console"/>
        </Logger>
        <Root level="debug">
          <AppenderRef ref="Console"/>
        </Root>
      </Loggers>
    </Configuration>

log4j update configuration during runtime, refresh configuration

<?xml version="1.0" encoding="UTF-8"?>
<Configuration monitorInterval="30">
...
</Configuration>

hsqldb Oracle dialect

driverClassName="org.hsqldb.jdbc.JDBCDriver"
url="jdbc:hsqldb:mem:test;sql.syntax_ora=true"
username="sa" password=""

liquibase

---

liquibase print sql scripts ( update sql )

java -jar C:\soft\maven-repo\org\liquibase\liquibase-core\3.4.1\liquibase-core-3.4.1.jar  \
--driver=oracle.jdbc.OracleDriver --classpath=C:\soft\maven-repo\com\oracle\ojdbc6\11.2.0.2.0\ojdbc6-11.2.0.2.0.jar  \
--changeLogFile=C:\temp\liquibase\brand-server\master.xml  \
--contexts="default" \
--url=jdbc:oracle:thin:@q-ora-db-scan.wirecard.sys:1521/stx11de.wirecard --username=horus_user_cherkavi --password=horus_user_cherkavi \
 updateSQL > script.sql

---

liquibase test scripts against H2 database

java -jar liquibase-core-3.5.5.jar  --driver=org.h2.Driver --classpath=h2-1.4.197.jar --changeLogFile=C:\project\opm\opm-persistence\src\main\resources\db\changelog\changes.xml --url=jdbc:h2:mem:testdb;Mode=Oracle --username=sa --password=  update

flyway

commandline documentation

folder structure

/-<root>
  /drivers
  /jars
  /scripts
  /sql

---

execute flyway from commandline, db info

java -cp flyway-core-4.0.3.jar;flyway-commandline-5.1.3.jar org.flywaydb.commandline.Main -configFile=flyway-info.conf info

---

to see debug log, need to add next jars into classpath:

• slf4j-api-1.7.25.jar
• log4j-to-slf4j-2.10.0.jar
• log4j-core-2.10.0.jar
• log4j-api-2.10.0.jar
• log4j-1.2.14.jar

---

slf4j logging

lib/log4j-core-2.11.0.jar:\
lib/log4j-api-2.11.0.jar:\
lib/log4j-slf4j-impl-2.11.0.jar:\

and execute

java -Dlog4j2.debug=true 

config file example

flyway.driver=oracle.jdbc.OracleDriver
flyway.url=jdbc:oracle:thin:@vpn050.kabel.de:1523:PMDR
flyway.user=login
flyway.password=pass
flyway.locations=filesystem:../fixnet-data/,classpath:db.migration.initialize

---

execute flyway with command line parameters

flyway migrate -driver=oracle.jdbc.OracleDriver -url=jdbc:oracle:thin:@vs050:1523:PMDR -user=xnet -password=xnet -locations=filesystem:/opt/oracle/scripts"

---

flyway table with current migration status

select * from {schema name}."schema_version";

---

update schema name with "success" flag

update {schema name}."schema_version" set "success"=1 where "version"='3.9'; 

---

update from java code, custom update from code,

package db.migration.initialize;
import org.flywaydb.core.api.migration.jdbc.JdbcMigration;
public class V1_9_9__insert_initial_gui_configurations implements JdbcMigration {
    @Override
    public void migrate(Connection connection) throws Exception
}

<location>classpath:db.migration.initialize</location>

---

flyway crc generation flyway control sum generator

    public static int calculateSum(String filePath) {

        final CRC32 crc32 = new CRC32();
 
        try (BufferedReader bufferedReader = new BufferedReader(new FileReader(new File(filePath))); ) {
            String line = null;
            while ((line = bufferedReader.readLine()) != null) {
		    line = BomFilter.FilterBomFromString(line);
	            crc32.update(StringUtils.trimLineBreak(line).getBytes(StandardCharsets.UTF_8));
            }
        } catch (IOException e) {
            System.err.printf("Error while trying to calculate CRC for file %s: %s\n", filePath, e.getMessage());
            System.exit(1);
        }
 
        int checksum = (int) crc32.getValue();
        return checksum;
    }

JNDI datasource examples:

   <Resource name="ds/JDBCDataSource" auth="Container"
              type="javax.sql.DataSource" 
              driverClassName="org.h2.Driver"
              url="jdbc:h2:~/testdb;Mode=Oracle"
              username="sa" 
              password="" maxActive="20" maxIdle="10"
              maxWait="-1"/>

   <Resource name="ds/JDBCDataSource" auth="Container"
              type="javax.sql.DataSource" 
              driverClassName="org.hsqldb.jdbc.JDBCDriver"
              url="jdbc:hsqldb:mem:test;sql.syntax_ora=true"
              username="sa" 
              password="sa" maxActive="20" maxIdle="10"
              maxWait="-1"/>

WildFly

check health of application

<host:port>/<application>/node

Java Testing

insert private fields

ReflectionUtils.makeAccessible(id);
ReflectionUtils.setField(id, brand, "myNewId");

order of tests execution

@FixMethodOrder

rest testing

RestAssured
body("brands.size()")

Test playground

Sonar

skip checking by code checker

// NOSONAR

@java.lang.SuppressWarnings("squid:S3655")

Hibernate

@Modifying

for custom queries only

force joining

@WhereJoinTable

inheritance types

- table per hierarchy ( SINGLE_TABLE )
- table per sub-class ( JOINED )
- table per class ( TABLE_PER_CLASS )

BPMN

• Activiti
• Flowable

Vaadin

custom components

spring integration

horizontal stepper, human progress line, to-do list

show frame

    void showFrame(File file){
        Window window = new Window();
        window.setWidth("90%");
        window.setHeight("90%");
        BrowserFrame e = new BrowserFrame("PDF File", new FileResource(file));
        e.setWidth("100%");
        e.setHeight("100%");
        window.setContent(e);
        window.center();
        window.setModal(true);
        UI.getCurrent().addWindow(window);
    }

debug window

http://localhost:8090/?debug

Derby

• tutorial
• admin page

into 'bin' folder add two variables for all scripts:

 export DERBY_INSTALL=/dev/shm/db/db-derby-10.14.2.0-bin/ 
 export DERBY_HOME=/dev/shm/db/db-derby-10.14.2.0-bin/ 

start with listening all incoming request ( not from localhost )

./startNetworkServer -h vldn338

create DB before using, from jdbc url

jdbc:derby://vldn338:1527/testDB;create=true

maven dependency

<dependency>
    <groupId>org.apache.derby</groupId>
    <artifactId>derbyclient</artifactId>
    <version>10.14.2.0</version>
</dependency>

jdbc

Driver: org.apache.derby.jdbc.ClientDriver
jdbc: jdbc:derby://vldn338:1527/dbName
user: <empty>
pass: <empty>

Activiti

user guide

eclipse plugin

additional documentation

create/init DB

activiti-engine-x.x.x.jar/org/activiti/db/create/activiti.create.sql

email, e-mail, smtp emulator

FakeSMTP

java -jar fakeSMTP.jar --help

run example:

java -jar fakeSMTP.jar -s -b -p 2525 -a 127.0.0.1  -o output_directory_name 

e-mail server smtp, pop3, imap with SSL

MailServer

java -Dgreenmail.smtp.hostname=0.0.0.0 -Dgreenmail.smtp.port=2525 -Dgreenmail.pop3.hostname=0.0.0.0 -Dgreenmail.pop3.port=8443 -Dgreenmail.users=Vitali.Cherkashyn:[email protected],user:[email protected] -jar  greenmail-standalone-1.5.7.jar 

WebLogic REST management

using REST services

---

list of all DataSources:

curl -X GET http://host:7001/management/wls/latest/datasources

---

return all accessible parameters for request

curl -X OPTION http://host:7001/management/wls/latest/datasources

---

request with authentication ( where magic string is "username:password" in base64 )

curl -H "Authorization: Basic d2VibG9naWM6d2VibG9naWMx" -X GET http://host:7001/management/wls/latest/datasources

---

request with authentication

curl --user username:password -X GET http://host:7001/management/wls/latest/datasources

---

description of one DataSource:

curl -X GET http://host:7001/management/wls/latest/datasources/id/{datasourceid}

---

domain runtime

curl --user weblogic:weblogic1  -X GET http://host:7001/management/weblogic/latest/domainRuntime

---

server parameters, server folders, server directories, server status, server classpath

curl --user weblogic:weblogic1  -X GET http://host:7001/management/weblogic/latest/domainRuntime/serverRuntimes

---

list of deployments

curl -v --user weblogic:weblogic1 -H X-Requested-By:MyClient -H Accept:application/json -X GET http://host:7001/management/wls/latest/deployments

---

list of applications

curl --user weblogic:weblogic1 -H Accept:application/json -X GET http://host:7001/management/wls/latest/deployments/application

---

info about one application

curl --user weblogic:weblogic1 -H Accept:application/json -X GET http://host:7001/management/wls/latest/deployments/application/id/userappname-gui-2018.02.00.00-SPRINT-7

---

remove application, undeploy application, uninstall application

curl --user weblogic:weblogic1 -H X-Requested-By:weblogic -H Accept:application/json -X DELETE http://host:7001/management/wls/latest/deployments/application/id/userappname-gui-2018.02.00.00-SPRINT-7

---

deploy application

curl -X POST --user weblogic:weblogic1 -H X-Requested-By:weblogic -H Accept:application/json -H Content-Type:application/json [email protected] http://host:7001/management/wls/latest/deployments/application

{
    "name":"userappname-gui-2018.02.00.00-SPRINT-7"
   ,"deploymentPath":"/opt/oracle/domains/pportal_dev/binaries/userappname-gui-2018.02.00.00-SPRINT-7.war"
   , "targets": ["pportal_group"]
}

OpenXava

OpenXava create new project from DataSource
	1 - switch to workspace %OPENXAVA%/workspace
	2 - open project OpenXavaTemplate
	3 - execute ant script: CreateNewProject.xml
	4 - enter name of project (Monolith) 
	5 - import new project into workspace ( import existing project )
	6 - replace dialect - %OPENXAVA%/workspace/Monolith/persistence/hibernate.cfg.xml
	7 - replace dialect - %OPENXAVA%/workspace/Monolith/persistence/META-INF/persistence.xml
	8 - create hibernate mapping from Database ( by JBoss tool )
	8.1 - create Hibernate console
	8.2 - create Hibernate Code Generation Configuration
	8.3 - run Hibernate Code Generation Configuration
	9 - add annotations:
	9.1 - add import to each Domain file: 
		import javax.persistence.*;
		import org.openxava.annotations.*;
	9.2 - add annotations to each column:
 	    @Id
	    // GeneratedValue(strategy=GenerationType.AUTO)
	    @Hidden
	    @Column(name = "value_id", length=36, nullable = false)
	    private String valueId;
	
    	    @Required
    	    @Column(name = "group", length = 20, nullable = false)
	    private String group;
	
	10 - add openxava annotations to each Entity:
	11 - execute ant compile ( %OPENXAVA%/workspace/Monolith/  )
	12 - execute ant deployWar ( %OPENXAVA%/workspace/Monolith/  )
	13 - execute tomcat
	14 - goto 
		http://localhost:8080/Monolith/modules/Value
		<     tomcat   path ><Project >       < entity name >

Version differences

jwt web token decoder JSON web token