How to Set Java Version in Linux

How to set java version in Linux or Unix.

Here I have used Linux Mint, but this setup can work for other Linux distributions. 

Checking the existing java version on your linux machine.
dev@developer-desktop ~ $ java -version
java version "1.6.0_36"
OpenJDK Runtime Environment (IcedTea6 1.13.8) (6b36-1.13.8-0ubuntu1~12.04)
OpenJDK 64-Bit Server VM (build 23.25-b01, mixed mode)

 
The above command shows the existing java version installed on this machine. Now I can check the alternative version to set. These alternative versions are available for installation. The below command will prompt with sudo password and selection number. Try with below command :


dev@developer-desktop ~ $ sudo update-alternatives --config java
[sudo] password for dev: ***********
There are 3 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                            Priority   Status
------------------------------------------------------------
* 0            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java   1061      auto mode
  1            /usr/bin/gij-4.6                                 1046      manual mode
  2            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java   1061      manual mode
  3            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1051      manual mode

Press enter to keep the current choice[*], or type selection number: 3
update-alternatives: using /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java to provide /usr/bin/java (java) in manual mode.

In the above command, I have opted the Selection 3 , because I want java 1.7 version. You can choose as per your requirements. 

Now you can check whether version 7 is installed or not ? Try the check version command again as below.

dev@developer-desktop ~ $ java -version
java version "1.7.0_79" -----> This is newly installed.
OpenJDK Runtime Environment (IcedTea 2.5.6) (7u79-2.5.6-0ubuntu1.12.04.1)
OpenJDK 64-Bit Server VM (build 24.79-b02, mixed mode)



Hope this will help you.

Spring Boot JAR Error: No Main Manifest Attribute

The error "no main manifest attribute, in sample-hello-microservice-springboot.jar" occurred while trying to execute the JAR file.

This was an unusual issue with JAR file execution. In this case, I was trying to run the Docker image. The reason for this issue was that, during JAR execution, it was not able to locate the main class.

Below is the main class, HelloApplication, and the pom.xml file. It seems that the Spring Boot Maven plugin was missing, along with the configuration for the manifest (mainClass).

HelloApplication.java

package com.techbyteslearn.lab.springboot.application;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class HelloApplication {

    public static void main(String[] args) {
        SpringApplication.run(HelloApplication.class, args);
    }
}

pom.xml

<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>sample-hello-microservice-springboot</groupId>
    <artifactId>sample-hello-microservice-springboot</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <description>Sample Hello microservice with Spring Boot.</description>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.1.RELEASE</version>
    </parent>

    <dependencies>

        <!-- Setup Spring Boot -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>

        <!-- Setup Spring MVC & REST with Embedded Tomcat -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

    </dependencies>

    <build>
        <sourceDirectory>src</sourceDirectory>

        <plugins>

            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>

            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <archive>
                        <manifest>
                            <mainClass>
                                com.techbyteslearn.lab.springboot.application.HelloApplication
                            </mainClass>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>

        </plugins>
    </build>

</project>

Solution:

I added the Spring Boot Maven plugin and configured the main class in the pom.xml file.

After making this change, the JAR was created with the required manifest information, and the application started working fine when the JAR was executed.

You can see the screenshot below to see the result.

Checking on browser.

Hope this will help you. 

JUnit Error: Unresolved Compilation Problems Due to Missing WebLogic JAR

java.lang.Error: Unresolved compilation problems:

Facing issue is often hurdle for developer :) :) As you all know, Java Developers Guide always posted the real time issues. This issue I have faced while writing Junit test cases, but its not so easy to fix your development desktop. The below issue normally happens when the class path is missed the related weblogic jar.

I am using weblogic for jaxrpc client calls, and its failing to find the weblogic jar. Below error logs :  

java.lang.Error: Unresolved compilation problems:
    The type weblogic.wsee.jaxrpc.ServiceImpl cannot be resolved. It is indirectly referenced from required .class files
    The type weblogic.wsee.jaxrpc.StubImpl cannot be resolved. It is indirectly referenced from required .class files
    The import weblogic.wsee.security cannot be resolved
    The import weblogic.xml cannot be resolved
    The import weblogic.xml cannot be resolved
    The method addCredentialsToSecurityHeader(Stub, HandlerInfo) in the type JdcNotificationHandler is not applicable for the arguments (JdcEnablerService_PortType_Stub, HandlerInfo)
    The method addCredentialsToSecurityHeader(Stub, HandlerInfo) in the type JdcNotificationHandler is not applicable for the arguments (JdcEnablerService_PortType_Stub, HandlerInfo)
    The method addCredentialsToSecurityHeader(Stub, HandlerInfo) in the type JdcNotificationHandler is not applicable for the arguments (JdcEnablerService_PortType_Stub, HandlerInfo)
    CredentialProvider cannot be resolved to a type
    CredentialProvider cannot be resolved to a type
    CredentialProvider cannot be resolved to a type
    ClientUNTCredentialProvider cannot be resolved to a type
    WSSecurityContext cannot be resolved to a variable

    at au.com.mycompany.mcas.sdp.bizservice.user.domain.util.notification.JdcNotificationHandler.<init>(JdcNotificationHandler.java:1)
    at au.com.mycompany.mcas.sdp.bizservice.user.application.facade.spring.handler.JdcNotificationHandlerTest.<init>(JdcNotificationHandlerTest.java:31)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at org.junit.internal.runners.JUnit4ClassRunner.createTest(JUnit4ClassRunner.java:72)
    at org.mockito.internal.runners.JUnit44RunnerImpl$1.createTest(JUnit44RunnerImpl.java:26)
    at org.junit.internal.runners.JUnit4ClassRunner.invokeTestMethod(JUnit4ClassRunner.java:79)
    at org.junit.internal.runners.JUnit4ClassRunner.runMethods(JUnit4ClassRunner.java:51)
    at org.junit.internal.runners.JUnit4ClassRunner$1.run(JUnit4ClassRunner.java:44)
    at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:27)
    at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:37)
    at org.junit.internal.runners.JUnit4ClassRunner.run(JUnit4ClassRunner.java:42)
    at org.mockito.internal.runners.JUnit44RunnerImpl.run(JUnit44RunnerImpl.java:37)
    at org.mockito.runners.MockitoJUnitRunner.run(MockitoJUnitRunner.java:62)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

How to fix  :- This is very simple, just add the weblogic.jar library jar into your class path. Steps as below,

Right Click on Project => Build Path => Libraries and then add the required jar (In my case I have added weblogic.jar).

The screenshot attached for reference. 

Eclipse Main Class Not Found: Troubleshooting Classpath Issues

This is the common issue with Eclipse , even I faced many times . This is not related to Eclipse or any other IDE tool.


This issue occurs when you are trying to run your main() method with respective class, its looking for the exact .class file in the class path.During the execution the .class file is not available in the class path. The best way to resolve this is, add the .class file into class path.

There are many possible diagnosis you can do in your Eclipse for this issue. As per the Eclipse behavior when you are updating the class path (Build Path) , your class is getting compiled that time and the .class file got created. So, Be careful when doing the build path for your project.

Also check your Build Path, if any jar file is missing. Because, its might causes sometime.

In my case I just found the jar file missing in the build path , and I removed the missing jar and added new jar file from other location.Then executed my class with main method and it works. Below screenshot shows my issue.

Java Command Not Working from Command Prompt in Windows

This issue I faced with window 8. When I was trying to execute java command from command prompt it was showing this error.After long time analysis I found the reason for this issue. This issue is really ambiguous.Its normally a path issue, if your class path has configured proper and still its not working then it might be a big reason to worry.

Solutions 1 :- Check your class path , if not set then use the below command to set.

path = your_java_installation_bin_path; 

Solutions 2 :- Probably there is few more java.exe file in your windows directory (i.e. C:\) .Normally when windows automatic updates works on your machine it may be install java into some other reference directory and it may be the cause.So, when you are trying to execute the java command from command prompt its not able to locate the exact path . 

 

As a solution , I have done this by removing all the java.exe files from windows directory.

You can search the java.exe file from windows directory and remove all java application files , except your own java installation directory. After removing all those extra java application files its working fine for me. Below the screen shot for finding and removing the java application file.