Lifecycle of a Java Program — Find the Bug¶

// File: HelloWorld.java
public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

// File: Main.java
public class Main {
    public static void main(String[] args) {
        System.out.println("Program running");
    }
}

public class Main {
    public void main(String[] args) {
        System.out.println("Application started");
    }
}

public class Main {
    static final int MAX_CONNECTIONS = MULTIPLIER * BASE;
    static final int BASE = 100;
    static final int MULTIPLIER = 2;

    public static void main(String[] args) {
        System.out.println("Max connections: " + MAX_CONNECTIONS);
    }
}

public class Main {
    static final int PORT;

    static {
        PORT = Integer.parseInt(System.getenv("APP_PORT"));
    }

    public static void main(String[] args) {
        System.out.println("Server starting on port: " + PORT);
    }
}

// Version 1: Main.java
public class Main {
    public static void main(String[] args) {
        System.out.println("Version 1");
    }
}

// Version 2: Main.java (updated)
public class Main {
    public static void main(String[] args) {
        System.out.println("Version 2 — with new features!");
    }
}

// Compiled with JDK 21
public class Main {
    public static void main(String[] args) {
        System.out.println("Running on Java " + Runtime.version());
    }
}

import java.io.FileWriter;
import java.io.IOException;

public class Main {
    static int counter = 0;

    public static void main(String[] args) throws InterruptedException {
        Runtime.getRuntime().addShutdownHook(new Thread(() -> {
            // Bug: counter may not have the final value
            try (FileWriter fw = new FileWriter("counter.txt")) {
                fw.write("Final count: " + counter);
                System.out.println("Saved counter: " + counter);
            } catch (IOException e) {
                System.err.println("Failed to save: " + e.getMessage());
            }
        }));

        for (int i = 0; i < 100; i++) {
            counter++;
            System.out.println("Count: " + counter);
            Thread.sleep(100);
        }
    }
}

import java.lang.ref.WeakReference;

public class Main {
    static ThreadLocal<Object> cache = new ThreadLocal<>();

    public static void main(String[] args) throws Exception {
        WeakReference<ClassLoader> clRef = loadAndUsePlugin();

        // Try to GC the ClassLoader
        System.gc();
        Thread.sleep(100);

        if (clRef.get() == null) {
            System.out.println("ClassLoader was garbage collected (no leak)");
        } else {
            System.out.println("ClassLoader LEAKED! Still in memory.");
        }
    }

    static WeakReference<ClassLoader> loadAndUsePlugin() throws Exception {
        // Simulate a custom ClassLoader
        ClassLoader pluginCL = new ClassLoader(Main.class.getClassLoader()) {
            @Override
            public Class<?> loadClass(String name) throws ClassNotFoundException {
                return super.loadClass(name); // Delegate to parent
            }
        };

        Class<?> stringClass = pluginCL.loadClass("java.lang.String");
        Object instance = "plugin-data";

        // Bug: Store object loaded by plugin's classloader in ThreadLocal
        cache.set(instance);

        return new WeakReference<>(pluginCL);
    }
}

import java.io.FileWriter;
import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        FileWriter writer = null;
        try {
            writer = new FileWriter("output.txt");
            writer.write("Processing data...\n");

            // Simulate a fatal error condition
            boolean fatalError = true;
            if (fatalError) {
                System.out.println("Fatal error detected. Exiting.");
                System.exit(1);  // Bug: finally block NEVER runs!
            }

            writer.write("Processing complete.\n");
        } catch (IOException e) {
            System.err.println("IO Error: " + e.getMessage());
        } finally {
            System.out.println("Finally: closing writer...");
            try {
                if (writer != null) {
                    writer.flush();
                    writer.close();
                }
            } catch (IOException e) {
                System.err.println("Error closing writer: " + e.getMessage());
            }
        }
    }
}

// File: A.java
public class A {
    static final int VALUE;
    static {
        System.out.println("A: initializing...");
        VALUE = B.VALUE + 1;  // Triggers B initialization
        System.out.println("A: VALUE = " + VALUE);
    }
}

// File: B.java
public class B {
    static final int VALUE;
    static {
        System.out.println("B: initializing...");
        VALUE = A.VALUE + 1;  // Triggers A initialization
        System.out.println("B: VALUE = " + VALUE);
    }
}

// File: Main.java
public class Main {
    public static void main(String[] args) {
        System.out.println("A.VALUE = " + A.VALUE);
        System.out.println("B.VALUE = " + B.VALUE);
    }
}