Java version detector

java-version.cpp

#include <filesystem>
#include <fstream>
#include <iostream>
#include <string>

namespace fs = std::filesystem;

int main(int argc, char* argv[]) {
    if (argc != 2) {
        std::cerr << "Usage: " << argv[0] << " <path-to-bin-java>\n";
        return 1;
    }

    fs::path javaPath;
    try {
        javaPath = fs::canonical(argv[1]);
    } catch (const fs::filesystem_error& e) {
        std::cerr << "Failed to resolve path: " << e.what() << "\n";
        return 1;
    }

    // Expect .../bin/java
    if (javaPath.filename() != "java" ||
        javaPath.parent_path().filename() != "bin") {
        std::cerr << "Path does not end with /bin/java\n";
        return 1;
    }

    // ../release
    fs::path releasePath = javaPath.parent_path().parent_path() / "release";

    std::ifstream file(releasePath);
    if (!file) {
        std::cerr << "Failed to open " << releasePath << "\n";
        return 1;
    }

    const std::string key = "JAVA_VERSION=\"";
    std::string line;

    while (std::getline(file, line)) {
        if (line.rfind(key, 0) == 0) { // starts with key
            auto start = key.size();
            auto end = line.find('"', start);
            if (end != std::string::npos) {
                std::cout << line.substr(start, end - start) << "\n";
                return 0;
            }
        }
    }

    std::cerr << "JAVA_VERSION not found\n";
    return 1;
}

This tool allows you to get the version of a java binary in a Java installation
in around one millisecond instead of > 20ms for java -version.

Compile it via:

g++ -std=c++17 java_version.cpp -o java_version

Then use it via:

./java-version `which java`
# or directly
./java-version java/25-sapmchn/bin/java

License

MIT