Deadlock in Java

// Deadlock happens due to circular waiting of locks
synchronized(obj1) {
  synchronized(obj2) {
    // critical section
  }
}

class A {

  synchronized void methodA(B b) {
    System.out.println("Thread A: holding lock of A");

    try { Thread.sleep(100); } catch (Exception e) {}

    System.out.println("Thread A: waiting for B");
    b.last();
  }

  synchronized void last() {
    System.out.println("Inside A last method");
  }

}

class B {

  synchronized void methodB(A a) {
    System.out.println("Thread B: holding lock of B");

    try { Thread.sleep(100); } catch (Exception e) {}

    System.out.println("Thread B: waiting for A");
    a.last();
  }

  synchronized void last() {
    System.out.println("Inside B last method");
  }

}

public class Main {

  public static void main(String[] args) {

    A a = new A();
    B b = new B();

    Thread t1 = new Thread(() -> a.methodA(b));
    Thread t2 = new Thread(() -> b.methodB(a));

    t1.start();
    t2.start();

  }

}

// Output (may freeze):
// Thread A: holding lock of A
// Thread B: holding lock of B
// Thread A: waiting for B
// Thread B: waiting for A

• 1 A situation where threads wait forever.
• 2 Caused by circular dependency of locks.
• 3 Stops program execution.
• 4 Hard to detect and debug.

• 1 Mutual exclusion.
• 2 Hold and wait.
• 3 No preemption.
• 4 Circular wait.

• 1 Maintain lock ordering.
• 2 Avoid multiple locks.
• 3 Use timeout-based locks.
• 4 Reduce synchronization scope.

• 1 Deadlock: threads stuck forever.
• 2 Starvation: thread waits too long.
• 3 Deadlock involves circular wait.
• 4 Starvation is scheduling issue.

• 1 Occurs in database systems with improper locking.
• 2 Seen in banking systems with multiple transactions.
• 3 Happens in distributed systems.
• 4 Common issue in multithreaded server applications.
• 5 SaaS products use Deadlock in Java in services, dashboards, background jobs, and API workflows.
• 6 ERP and banking systems apply Deadlock in Java with validation, logging, review, and rollback plans.
• 7 E-commerce and healthcare platforms use Deadlock in Java carefully because reliability and data correctness matter.

• 1 A Java program first evaluates the surrounding context, then applies the Deadlock in Java rules to the current data.
• 2 The important mental model is input, transformation, result, and failure path.
• 3 In production, the same flow usually sits inside a larger layer such as a controller, service, repository, job, or UI component.

• 1 Choose the simplest implementation first, then measure real workloads.
• 2 Watch for repeated work inside loops, unnecessary allocations, and slow I/O in hot paths.
• 3 Prefer clear data structures and stable APIs before micro-optimizing syntax.

• 1 Treat external input as untrusted until it is validated.
• 2 Avoid hardcoded secrets and never print sensitive values in examples or logs.
• 3 Use established libraries for authentication, encryption, parsing, and database access.

• 1 Acquiring multiple locks in different order.
• 2 Not using timeout mechanisms.
• 3 Ignoring lock hierarchy design.
• 4 Overusing synchronized blocks.
• 5 Skipping the small working example before adding framework code.
• 6 Ignoring null, empty, duplicate, and boundary inputs.
• 7 Mixing business logic, input handling, and output formatting in one place.
• 8 Using broad error handling that hides the real failure.
• 9 Forgetting to test the behavior after refactoring.
• 10 Adding clever code that future maintainers will struggle to read.

• 1 Always acquire locks in same order.
• 2 Avoid nested synchronized blocks.
• 3 Use tryLock() where possible.
• 4 Keep critical sections small.
• 5 Start with clear requirements and one minimal working example.
• 6 Use meaningful names that explain business intent.
• 7 Keep examples small enough to debug line by line.
• 8 Validate input at every trust boundary.
• 9 Handle errors explicitly and preserve useful context.
• 10 Prefer simple control flow over deeply nested logic.
• 11 Separate domain logic from I/O and framework code.
• 12 Write tests for normal, boundary, and failure cases.
• 13 Review security assumptions before production use.
• 14 Measure performance before optimizing.
• 15 Document non-obvious decisions close to the code or in project notes.
• 16 Use official documentation when behavior is version-specific.
• 17 Keep dependencies current and remove unused code.
• 18 Avoid hardcoded secrets, credentials, and environment-specific paths.
• 19 Log operational events without exposing sensitive data.
• 20 Design examples so learners can safely modify and rerun them.

• 1 Beginner: rewrite the example with different names and values.
• 2 Intermediate: add validation and handle one expected failure case.
• 3 Advanced: place Deadlock in Java inside a small service-style design with tests.

• 1 Build a small Java console feature that demonstrates Deadlock in Java.
• 2 Accept input, process it with the concept, print a clear result, and handle invalid input.
• 3 Add a README note explaining the design choice and two edge cases you tested.

• 1 If the program does not compile, check spelling, imports, braces, and file/class names first.
• 2 If output is unexpected, print intermediate values and verify each branch of the logic.
• 3 If the design feels complex, reduce it to the smallest working example and add pieces back one at a time.

• 1 Practice Deadlock in Java with a second example from a business domain such as inventory, payroll, banking, or e-commerce.
• 2 Review related Java topics that cover data flow, error handling, testing, and clean design.
• 3 Compare your solution with official documentation and simplify anything you cannot explain clearly.