Use static fields and properties from JAR library in C++ app

This article provides an introduction to cross-technology handling of static fields and properties.
Javonet allows you to reference and use modules or packages written in (Java/Kotlin/Groovy/Clojure, C#/VB.NET, Ruby, Perl, Python, JavaScript/TypeScript) like they were created in your technology. If have not yet created your first project check Javonet overview and quick start guides for your technology.

With Javonet you can interact with static field and properties from JAR library like they were available in C++ but invocation must be performed through Javonet SDK API.

Custom JAR library with static fields and properties

With Javonet it is possible to reference any custom JAR library and interact with public static fields and properties declared on types defined within that module almost the same as with any other C++ library.

Snippet below represents the sample code from JAR library which contains class with fields:

public class TestClass {
    public TestClass() { }

        public int publicValue;
        private int privateValue;
        public static int staticValue = 3;
        
        public TestClass(int publicValue, int privateValue) {
            this.publicValue = publicValue;
            this.privateValue = privateValue;
        }
}
public int publicValue;
private int privateValue;
public static int staticValue = 3;

public TestClass(int publicValue, int privateValue) {
    this.publicValue = publicValue;
    this.privateValue = privateValue;
}

It is possible to get one of the declared static fields from JAR library using following C++ code.

#include "Javonet.h"
#include <iostream>
#include <vector>

using namespace JavonetNS::Cpp::Sdk;

std::string resourcesDirectory = ".";

int main(int argc, char* argv[]) {

    // use Activate only once in your app
    Javonet::Activate("your-license-key");
    
    // create called runtime context
    auto calledRuntime = Javonet::InMemory()->Jvm();
    
    // set up variables
    auto libraryPath = resourcesDirectory + "/TestClass.jar";
    auto className = "TestClass";
    
    // load custom library
    calledRuntime->LoadLibrary(libraryPath);
    
    // get type from the runtime
    auto calledRuntimeType = calledRuntime->GetType(className)->Execute();
    
    // get type's static field
    auto response = calledRuntimeType->GetStaticField("staticValue")->Execute();
    
    // get value from response
    auto result = std::any_cast<int>(response->GetValue());
    
    // write result to console
    std::cout << result << std::endl;
}public class TestClass {
    public TestClass() { }

        public int publicValue;
        private int privateValue;
        public static int staticValue = 3;
        
        public TestClass(int publicValue, int privateValue) {
            this.publicValue = publicValue;
            this.privateValue = privateValue;
        }
}
// use Activate only once in your app
Javonet::Activate("your-license-key");

// create called runtime context
auto calledRuntime = Javonet::InMemory()->Jvm();

// set up variables
auto libraryPath = resourcesDirectory + "/TestClass.jar";
auto className = "TestClass";

// load custom library
calledRuntime->LoadLibrary(libraryPath);

// get type from the runtime
auto calledRuntimeType = calledRuntime->GetType(className)->Execute();

// get type's static field
auto response = calledRuntimeType->GetStaticField("staticValue")->Execute();

// get value from response
auto result = std::any_cast<int>(response->GetValue());

// write result to console
std::cout << result << std::endl;

This uses in memory runtime bridging to load the JAR library, and next retrieves reference to specific type and gets the static field. Result of the invocation is returned as regular C++ value and can be used for further processing. It is possible to set one of the declared static fields from JAR library using following C++ code.

#include "Javonet.h"
#include <iostream>
#include <vector>

using namespace JavonetNS::Cpp::Sdk;

std::string resourcesDirectory = ".";

int main(int argc, char* argv[]) {

    // use Activate only once in your app
    Javonet::Activate("your-license-key");
    
    // create called runtime context
    auto calledRuntime = Javonet::InMemory()->Jvm();
    
    // set up variables
    auto libraryPath = resourcesDirectory + "/TestClass.jar";
    auto className = "TestClass";
    
    // load custom library
    calledRuntime->LoadLibrary(libraryPath);
    
    // get type from the runtime
    auto calledRuntimeType = calledRuntime->GetType(className)->Execute();
    
    // set type's static field
    calledRuntimeType->SetStaticField("staticValue", 75)->Execute();
    
    // get type's static field
    auto response = calledRuntimeType->GetStaticField("staticValue")->Execute();
    
    // get value from response
    auto result = std::any_cast<int>(response->GetValue());
    
    // write result to console
    std::cout << result << std::endl;
}public class TestClass {
    public TestClass() { }

        public int publicValue;
        private int privateValue;
        public static int staticValue = 3;
        
        public TestClass(int publicValue, int privateValue) {
            this.publicValue = publicValue;
            this.privateValue = privateValue;
        }
}
// use Activate only once in your app
Javonet::Activate("your-license-key");

// create called runtime context
auto calledRuntime = Javonet::InMemory()->Jvm();

// set up variables
auto libraryPath = resourcesDirectory + "/TestClass.jar";
auto className = "TestClass";

// load custom library
calledRuntime->LoadLibrary(libraryPath);

// get type from the runtime
auto calledRuntimeType = calledRuntime->GetType(className)->Execute();

// set type's static field
calledRuntimeType->SetStaticField("staticValue", 75)->Execute();

// get type's static field
auto response = calledRuntimeType->GetStaticField("staticValue")->Execute();

// get value from response
auto result = std::any_cast<int>(response->GetValue());

// write result to console
std::cout << result << std::endl;

This uses in memory runtime bridging to load the JAR library, and next retrieves reference to specific type, then sets and gets the static field. Result of the invocation is returned as regular C++ value and can be used for further processing.

Framework static fields and properties

The same steps are required to get types and fields from framework JAR library:

#include "Javonet.h"
#include <iostream>
#include <vector>

using namespace JavonetNS::Cpp::Sdk;

std::string resourcesDirectory = ".";

int main(int argc, char* argv[]) {

    // use Activate only once in your app
    Javonet::Activate("your-license-key");
    
    // create called runtime context
    auto calledRuntime = Javonet::InMemory()->Jvm();
    
    // get type from the runtime
    auto calledRuntimeType = calledRuntime->GetType("java.lang.Math")->Execute();
    
    // get type's static field
    auto response = calledRuntimeType->GetStaticField("PI")->Execute();
    
    // get value from response
    auto result = std::any_cast<double>(response->GetValue());
    
    // write result to console
    std::cout << result << std::endl;
}public class TestClass {
    public TestClass() { }

        // empty
}
// use Activate only once in your app
Javonet::Activate("your-license-key");

// create called runtime context
auto calledRuntime = Javonet::InMemory()->Jvm();

// get type from the runtime
auto calledRuntimeType = calledRuntime->GetType("java.lang.Math")->Execute();

// get type's static field
auto response = calledRuntimeType->GetStaticField("PI")->Execute();

// get value from response
auto result = std::any_cast<double>(response->GetValue());

// write result to console
std::cout << result << std::endl;

The same operation can be performed remotely by just changing the new Runtime Context invocation from in memory to tcp that will create and interact with your JAR library objects on any remote node, container or service that hosts Javonet Code Gateway. This way you can preserve the same logic in your application and instantly switch between monolithic and microservices architecture without the need to implement the integration layer based on web services or other remote invocation methods.

Read more about use cases and software architecture scenarios where Javonet runtime bridging technology can support your development process.

Use static fields and properties from JAR library in C++ app

Custom JAR library with static fields and properties

Framework static fields and properties

Additional references