InvalidOperationException: Can't use schemaId for type The same schemaId is already used for type

 After adding new bean to project I got below error during startup

13:44:19:254 ---> System.InvalidOperationException: Can't use schemaId "$Employee" for type "$staff.Employee". The same schemaId is already used for type "$test.Employee" 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaRepository.RegisterType(Type type, String schemaId) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateReferencedSchema(DataContract dataContract, SchemaRepository schemaRepository, Func`1 definitionFactory) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateConcreteSchema(DataContract dataContract, SchemaRepository schemaRepository) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchemaForType(Type modelType, SchemaRepository schemaRepository) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchema(Type modelType, SchemaRepository schemaRepository, MemberInfo memberInfo, ParameterInfo parameterInfo) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.CreateArraySchema(DataContract dataContract, SchemaRepository schemaRepository) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.<>c__DisplayClass10_0.b__1() 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateConcreteSchema(DataContract dataContract, SchemaRepository schemaRepository) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchemaForParameter(Type modelType, SchemaRepository schemaRepository, ParameterInfo parameterInfo) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchema(Type modelType, SchemaRepository schemaRepository, MemberInfo memberInfo, ParameterInfo parameterInfo) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateSchema(Type type, SchemaRepository schemaRepository, PropertyInfo propertyInfo, ParameterInfo parameterInfo) 13:44:19:254 --- End of inner exception stack trace --- 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateSchema(Type type, SchemaRepository schemaRepository, PropertyInfo propertyInfo, ParameterInfo parameterInfo) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateRequestBodyFromBodyParameter(ApiDescription apiDescription, SchemaRepository schemaRepository, ApiParameterDescription bodyParameter) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateRequestBody(ApiDescription apiDescription, SchemaRepository schemaRepository) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperation(ApiDescription apiDescription, SchemaRepository schemaRepository) 13:44:19:254 --- End of inner exception stack trace --- 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperation(ApiDescription apiDescription, SchemaRepository schemaRepository) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperations(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GeneratePaths(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository) 13:44:19:254 at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwagger(String documentName, String host, String basePath) 13:44:19:254 at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider) 13:44:19:254 at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context) 13:44:19:254 at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

Solution : 

The only change needed is in your Startup.cs inside the method ConfigureServices.

You should add the following:

services.AddSwaggerGen(options =>

{

    options.CustomSchemaIds(type => type.ToString());

});

How do I fix dependency conflicts in npm install?

Today I was working on a front end app. I faced below error while running npm install command.

npm ERR! Conflicting peer dependency: @angular/core@6.1.10

npm ERR! node_modules/@angular/core

npm ERR!   peer @angular/core@"^6.0.3" from @angular-redux/router@9.0.0

npm ERR!   node_modules/@angular-redux/router

npm ERR!     @angular-redux/router@"^9.0.0" from the root project

Fix for Conflicting peer dependency in node.js

• Downgrade the npm version to the previous version.
• remove the node_modules folder.
• remove package-lock.json.
• please do npm install one more time to do a fresh installation of the dependency.

Detecting a active transaction in spring application

Sometimes we need to use transactions (@Transactional) in code. We use @Transactional annotation on class and method to generate transactions.

In order to troubleshoot transaction issues, it could be helpful to verify if we have an active transaction in the current method or not.

Spring has provided a class called TransactionSychronizationManager. This class has a static method that allows us to know whether we are in a transaction (active transaction or no transaction), called isActualTransactionActive().

You can call this method and print output in logs. It will be true for active transactions false otherwise.

For example :

logger.error("Transaction is active " + TransactionSynchronizationManager.isActualTransactionActive());

Microservices Best Practices for the Optimal Architecture Design

Microservices have fundamentally changed the way server side applications are architected. Rather than a single giant monolithic codebase hosting all the business logic of your application, microservices reflect the distributed systems model, where a group of application components work together to deliver the business needs.

1. The Single Responsibility Principle

Just like with code, where a class should have only a single reason to change, microservices should be modeled in a similar fashion. Building bloated services which are subject to change for more than one business context is a bad practice.

2. Have a separate data store(s) for your microservice

It defeats the purpose of having microservices if you are using a monolithic database that all your microservices share. Any change or downtime to that database would then impact all the microservices that use the database. Choose the right database for your microservice needs, customize the infrastructure and storage to the data that it maintains, and let it be exclusive to your microservice. Ideally, any other microservice that needs access to that data would only access it through the APIs that the microservice with write access has exposed. 

3. Use asynchronous communication to achieve loose coupling

To avoid building a mesh of tightly coupled components, consider using asynchronous communication between microservices. 

4. Fail fast by using a circuit breaker to achieve fault tolerance

If your microservice is dependent on another system to provide a response, and that system takes forever to respond, your overall response SLAs will be impacted. To avoid this scenario and quickly respond, one simple microservices best practice you can follow is to use a circuit breaker to timeout the external call and return a default response or an error. This will isolate the failing services that your service is dependent on without causing cascade failures, keeping your microservice in good health. You can choose to use popular products like Hystrix that Netflix developed.

5. Proxy your microservice requests through an API Gateway

Instead of every microservice in the system performing the functions of API authentication, request / response logging, and throttling, having an API gateway doing these for you upfront will add a lot of value. Clients calling your microservices will connect to the API Gateway instead of directly calling your service. This way you will avoid making all those additional calls from your microservice and the internal URLs of your service would be hidden, giving you the flexibility to redirect the traffic from the API Gateway to a newer version of your service. This is even more necessary when a third party is accessing your service, as you can throttle the incoming traffic and reject unauthorized requests from the API gateway before they reach your microservice. You can also choose to have a separate API gateway that accepts traffic from external networks. 

6. Ensure your API changes are backwards compatible

You can safely introduce changes to your API and release them fast as long as they don’t break existing callers. 

7. Version your microservices for breaking changes

It's not always possible to make backwards compatible changes. When you are making a breaking change, expose a new version of your endpoint while continuing to support older versions. Consumers can choose to use the new version at their convenience. However, having too many versions of your API can create a nightmare for those maintaining the code. Hence, have a disciplined approach to deprecate older versions by working with your clients or internally rerouting the traffic to the newer versions.

8. Have dedicated infrastructure hosting your microservice

You can have the best designed microservice meeting all the checks, but with a bad design of the hosting platform it would still behave poorly. Isolate your microservice infrastructure from other components to get fault isolation and best performance. It is also important to isolate the infrastructure of the components that your microservice depends on.

9. Create a separate release train

Your microservice needs to have its own separate release vehicle which is not tied to other components within your organization. This way you are not stepping on each other’s toes and wasting time coordinating with multiple teams.

10. Create Organizational Efficiencies

While microservices give you the freedom to develop and release independently, certain standards need to be followed for cross cutting concerns so that every team doesn’t spend time creating unique solutions for these. This is very important in a distributed architecture such as microservices, where you need to be able to connect all the pieces of the puzzle to see a holistic picture. Hence, enterprise solutions are necessary for API security, log aggregation, monitoring, API documentation, secrets management, config management, distributed tracing, etc. 

By following these microservices best practices, you should end up with a loosely coupled, distributed, and independent microservice system, wherein you can achieve the true benefits of a microservices architecture as listed at the beginning of this article.

Remove a file from git pull request

Learned something amazing today.

Scenario : You have updated a file that already existed in main branch and want to remove it from the pull request.

Assume you're working on a branch (say featureBranch) off of development (means you created featureBranch from the development  branch) , and your pull request includes a file named AccountController.java.

To remove this file from the pull request and revert its changes, simply do :

1. Checkout your branch (featureBranch).
2. Run below command.
     git checkout development --  AccountController.java
3. Commit and push into your branch (featureBranch).

IntelliJ | SLF4J: Failed to load class org.slf4j.impl.StaticLoggerBinder

JAVA standalone program failed with below error on IntelliJ (Community Edition 2020.3).

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".

SLF4J: Defaulting to no-operation (NOP) logger implementation

SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

Process finished with exit code 0

Fix : 

Go to File-> Project Structure -> Modules,  then update slf4j dependency to compile. 

Java 14 Features with examples

Java 14, another non-LTS version has been released on March 17, 2020. Let's discuss it's feature with examples.

Here’s the list of Java 14 features:

• Pattern Matching for instanceof (Preview) – JEP 305
• Switch Expressions (Standard) – JEP 361
• Helpful NullPointerExceptions – JEP 358
• Records (Preview) – JEP 359
• Text Blocks (Second Preview) – JEP 368
• Packaging Tool (Incubator) – JEP 343
• NUMA-Aware Memory Allocation for G1 – JEP 345
• JFR Event Streaming – JEP 349
• Non-Volatile Mapped Byte Buffers – JEP 352
• ZGC on macOS – JEP 364
• ZGC on Windows – JEP 365
• Foreign-Memory Access API (Incubator) – JEP 370

1. Pattern Matching for instanceof (Preview) : Before Java 14, we use instanceof and cast to check the object’s type and cast to a variable. For example.

if (obj instanceof String) { // instanceof
      String str = (String) obj; // cast after type check
      if("test".equals(str)){
      }
} else {
     System.out.println("value is not a string");
}

Now using Java 14, we can refactor above code like this.if obj is an instance of String, then it is cast to String and assigned to the binding variable str.

if (obj instanceof String str) { // instanceof, cast and bind the variable
    if("test".equals(str)){
    }
} else {
    System.out.println("value is not a string");
}

2.  Switch Expressions (Standard) – JEP 361 : The switch expression was a preview feature in Java 12 and Java 13; now it is officially JDK standard feature, it means from Java 14 and onward, we can return value via switch expressions (we can use yield or label rules).

private int getSwitchValueViaYield(String mode) {

        int result = switch (mode) {

            case "Developer", "Tester":

                yield 10;

            case "Lead":

                yield 20;

            case "CTO", "CEO", "Director":

System.out.println("Supports multi line block!");

                yield 30;

            default:

                yield -1;

        };

        return result;

 }

3. Helpful NullPointerExceptions – JEP 358 : Improved the description of NullPointerExceptions by telling which variable was null. Add -XX:+ShowCodeDetailsInExceptionMessages option to enable this feature.

public class NullPointerExample {

    public static void main(String[] args) {
        String input = null;
        String result = showLowerCase(input); 
        System.out.println(result);
    }

    public static String showLowerCase(String str){
        return str.toLowerCase();
    }
}

Before JAVA 14 :

D:\Learning\JAVA 14\src>java com/blogspot/edgydebug/NullPointerExample.java
Exception in thread "main" java.lang.NullPointerException
        at com.blogspot.edgydebug.NullPointerExample.showLowerCase(NullPointerExample.java:12)

        at com.blogspot.edgydebug.NullPointerExample.main(NullPointerExample.java:7)



With JAVA 14 (-XX:+ShowCodeDetailsInExceptionMessages) :

D:\Learning\JAVA 14\src>java -XX:+ShowCodeDetailsInExceptionMessages com/blogspot/edgydebug/NullPointerExample.java
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.toLowerCase()" because "<parameter1>" is null
at com.blogspot.edgydebug.NullPointerExample.showLowerCase(NullPointerExample.java:12)
at com.blogspot.edgydebug.NullPointerExample.main(NullPointerExample.java:7)

4. Records (Preview) – JEP 359 : This feature is introduced to simplify the process of custom class creation. With a record you need to simply define a class in the following way.
Student.java :

package com.blogspot.edgydebug;
public record Student(String name, String id) {}

RecordExample.java :

package com.blogspot.edgydebug;

public class RecordExample {

     public static void main(String[] args) {
          Student student = new Student("John","12AWS34");
          System.out.println(student.name() + " "+student.id());
     }

}

D:\Learning\JAVA 14\src>javac --enable-preview --source 14 com\blogspot\edgydebug\Student.java
Note: com\blogspot\edgydebug\Student.java uses preview language features.
Note: Recompile with -Xlint:preview for details.

D:\Learning\JAVA 14\src>java --enable-preview --source 14 com\blogspot\edgydebug\RecordExample.java
Note: com\blogspot\edgydebug\RecordExample.java uses preview language features.
Note: Recompile with -Xlint:preview for details.
John 12AWS34

• The Java compiler will generate a constructor, private final fields, accessors, equals/hashCode and toString methods automatically.
• Points related to record class :
• Records cannot be abstract
• A record can neither extend a class nor it can be extended by another class. It’s a final class.
• Records cannot extend any other class and cannot define instance fields inside the body. Instance fields must be defined in the state description only
• The body of a record allows static fields and methods
• Declared fields are private and final

5. Text Blocks (Second Preview) – JEP 368 : Text Blocks were introduced as a preview feature in Java 13. The text block helps easy creation of multiline string literals. It’s useful in easily creating HTML and JSON or SQL query strings.

For example :
package com.blogspot.edgydebug;
public class TextBlockExample {
      public static void main(String[] args) {
                String text = """
                                    <html>
                                   <body>\
                                   <p>Hello, '\s' World</p>\
                                  </body>
                                  </html>
                                  """;
               System.out.println(text);
   }
}

Output :
D:\Learning\JAVA 14\src>java --enable-preview --source 14 com\blogspot\edgydebug\TextBlockExample.java
Note: com\blogspot\edgydebug\TextBlockExample.java uses preview language features.
Note: Recompile with -Xlint:preview for details.
<html>
<body> <p>Hello, ' ' World</p> </body>
</html>

6. Packaging Tool (Incubator) – JEP 343 :

New jpackage tool to package a Java application into a platform-specific package.
Linux: deb and rpm
macOS: pkg and dmg
Windows: msi and exe (package the JAR file into an exe file on Windows)

7. NUMA-Aware Memory Allocation for G1 – JEP 345 : New NUMA-aware memory allocation mode, improves the G1 performance on large machines.Add +XX:+UseNUMA option to enable it.

8. JFR Event Streaming – JEP 349 : Improved the existing JFR (JDK Flight Recorder) to support event streaming, it means now we can stream the JFR events in real-time, without the need to dump the recorded events to disk and parse it manually.

9. Non-Volatile Mapped Byte Buffers – JEP 352 : Improved FileChannel API to create MappedByteBuffer that access to non-volatile memory (NVM)  a memory that can retrieve stored data even after having been power cycled. For example, this feature ensures that any changes which might still be in the cache are written back to memory.

10. ZGC on macOS – JEP 364 : Java 11 – JEP 333 introduced the Z Garbage Collector (ZGC) on Linux, and now it ports to macOS.

11. ZGC on Windows – JEP 365 : Java 11 – JEP 333 introduced the Z Garbage Collector (ZGC) on Linux, and now it ports to Windows version >= 1803.

12. Foreign-Memory Access API (Incubator) – JEP 370 : Introduce an API to allow Java programs to safely and efficiently access foreign memory outside of the Java heap.

References : https://openjdk.java.net/projects/jdk/14/