Error Handling: Exceptions, Try-with-Resources, and Result Patterns

Good error handling is the difference between an application that crashes mysteriously and one that fails gracefully with actionable information. This page covers Java's exception model, resource management, and functional alternatives.

Exception hierarchy

Type	Checked?	Catch required?	When to use
Error	No	No (don't catch)	JVM problems (OutOfMemory, StackOverflow)
Exception (checked)	Yes	Yes	Recoverable failures (IO, SQL, network)
RuntimeException (unchecked)	No	Optional	Programming errors, validation failures

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Checked vs unchecked exceptions

Checked exceptions

The compiler forces you to handle them (try/catch or declare throws):

// Must handle IOException
public String readFile(Path path) throws IOException {
    return Files.readString(path);
}

// Caller option 1: catch
try {
    String content = readFile(Path.of("data.txt"));
} catch (IOException e) {
    log.error("Failed to read file", e);
    return "default";
}

// Caller option 2: propagate
public void process() throws IOException {
    String content = readFile(Path.of("data.txt"));
    // ...
}

Unchecked exceptions

No compiler enforcement -- they indicate programming errors or precondition violations:

public void setAge(int age) {
    if (age < 0 || age > 150) {
        throw new IllegalArgumentException("Age must be 0-150, got: " + age);
    }
    this.age = age;
}

public void withdraw(BigDecimal amount) {
    if (amount.compareTo(balance) > 0) {
        throw new IllegalStateException(
            "Insufficient balance: " + balance + ", requested: " + amount);
    }
    balance = balance.subtract(amount);
}

When to use which

Use checked exceptions	Use unchecked exceptions
Caller can reasonably recover	Programming error (bug)
External system failure (I/O, network, DB)	Validation failure (bad input)
Unusual but expected condition	Invariant violation
API boundary (forces caller to think about failure)	Internal logic errors

Modern Java style leans toward unchecked exceptions for most application-level errors, reserving checked exceptions for truly recoverable I/O operations.

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Try-with-resources

Automatically closes AutoCloseable resources when the block exits (normally or via exception):

// Single resource
try (var reader = new BufferedReader(new FileReader("data.txt"))) {
    String line = reader.readLine();
}
// reader is closed automatically, even if an exception occurs

// Multiple resources
try (var conn = dataSource.getConnection();
     var stmt = conn.prepareStatement("SELECT * FROM users");
     var rs = stmt.executeQuery()) {

    while (rs.next()) {
        System.out.println(rs.getString("name"));
    }
}
// rs, stmt, and conn are closed in reverse order

Implementing AutoCloseable

public class DatabasePool implements AutoCloseable {
    private final List<Connection> connections = new ArrayList<>();

    public Connection acquire() { /* ... */ }

    @Override
    public void close() {
        connections.forEach(conn -> {
            try { conn.close(); }
            catch (SQLException e) { log.warn("Failed to close connection", e); }
        });
        connections.clear();
    }
}

// Usage
try (var pool = new DatabasePool()) {
    Connection conn = pool.acquire();
    // ...
} // pool.close() called automatically

Suppressed exceptions

If both the try block and the close() method throw, the close exception is suppressed (attached to the primary exception):

try (var resource = new FailingResource()) {
    throw new RuntimeException("primary");
} catch (RuntimeException e) {
    System.out.println(e.getMessage()); // "primary"
    for (Throwable suppressed : e.getSuppressed()) {
        System.out.println("Suppressed: " + suppressed.getMessage());
    }
}

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Custom exception hierarchies

Base exception

public abstract class AppException extends RuntimeException {
    private final String errorCode;

    protected AppException(String errorCode, String message) {
        super(message);
        this.errorCode = errorCode;
    }

    protected AppException(String errorCode, String message, Throwable cause) {
        super(message, cause);
        this.errorCode = errorCode;
    }

    public String getErrorCode() { return errorCode; }
}

Specific exceptions

public class NotFoundException extends AppException {
    public NotFoundException(String entity, String id) {
        super("NOT_FOUND", entity + " not found: " + id);
    }
}

public class ValidationException extends AppException {
    private final Map<String, String> fieldErrors;

    public ValidationException(Map<String, String> fieldErrors) {
        super("VALIDATION_ERROR", "Validation failed: " + fieldErrors);
        this.fieldErrors = Map.copyOf(fieldErrors);
    }

    public Map<String, String> getFieldErrors() { return fieldErrors; }
}

public class ExternalServiceException extends AppException {
    public ExternalServiceException(String service, Throwable cause) {
        super("EXTERNAL_ERROR", "Call to " + service + " failed", cause);
    }
}

Usage

public User findUser(String id) {
    return userRepository.findById(id)
        .orElseThrow(() -> new NotFoundException("User", id));
}

public void createUser(UserRequest request) {
    Map<String, String> errors = validate(request);
    if (!errors.isEmpty()) {
        throw new ValidationException(errors);
    }
    // ...
}

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Multi-catch and finally

try {
    process();
} catch (IOException | SQLException e) {
    // Multi-catch: handle both the same way
    log.error("I/O or DB error", e);
} catch (AppException e) {
    // App-specific handling
    log.warn("Application error: {}", e.getErrorCode());
} finally {
    // Always runs (even if exception is thrown or caught)
    cleanup();
}

Prefer try-with-resources over finally for resource cleanup. Use finally only for non-resource cleanup logic.

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Result / Either pattern

A functional alternative to exceptions for expected failure cases. The method returns a value that explicitly represents success or failure:

Simple Result type

sealed interface Result<T> permits Result.Success, Result.Failure {

    record Success<T>(T value) implements Result<T> {}
    record Failure<T>(String error) implements Result<T> {}

    static <T> Result<T> success(T value) { return new Success<>(value); }
    static <T> Result<T> failure(String error) { return new Failure<>(error); }

    default <R> Result<R> map(Function<T, R> fn) {
        return switch (this) {
            case Success<T> s -> Result.success(fn.apply(s.value()));
            case Failure<T> f -> Result.failure(f.error());
        };
    }

    default <R> Result<R> flatMap(Function<T, Result<R>> fn) {
        return switch (this) {
            case Success<T> s -> fn.apply(s.value());
            case Failure<T> f -> Result.failure(f.error());
        };
    }

    default T orElse(T defaultValue) {
        return switch (this) {
            case Success<T> s -> s.value();
            case Failure<T> f -> defaultValue;
        };
    }
}

Usage

Result<User> findUser(String email) {
    if (email == null || email.isBlank()) {
        return Result.failure("Email must not be blank");
    }
    return userRepo.findByEmail(email)
        .map(Result::success)
        .orElse(Result.failure("User not found: " + email));
}

// Chain operations
Result<String> greeting = findUser("alice@example.com")
    .map(User::name)
    .map(name -> "Hello, " + name);

// Handle both cases
switch (greeting) {
    case Result.Success<String> s -> System.out.println(s.value());
    case Result.Failure<String> f -> System.err.println("Error: " + f.error());
}

When to use Result vs Exceptions

Use Result	Use Exceptions
Expected failure (validation, not-found)	Unexpected failure (bug, infrastructure)
Caller should handle inline	Error should propagate up the stack
Functional pipelines	Traditional imperative code
You want exhaustive handling at compile time	You want a global error handler

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Logging exceptions properly

// GOOD: pass the exception as the last argument (SLF4J prints the stack trace)
try {
    riskyOperation();
} catch (Exception e) {
    log.error("Operation failed for user {}", userId, e);
}

// BAD: toString() only -- loses the stack trace
log.error("Operation failed: " + e.toString());

// BAD: getMessage() only -- loses the exception type and stack trace
log.error("Operation failed: {}", e.getMessage());

// BAD: manual stack trace printing (use the logger instead)
e.printStackTrace();

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Exception handling in streams

Streams do not support checked exceptions. Workarounds:

Wrapping in unchecked

@FunctionalInterface
interface ThrowingFunction<T, R> {
    R apply(T t) throws Exception;

    static <T, R> Function<T, R> unchecked(ThrowingFunction<T, R> fn) {
        return t -> {
            try { return fn.apply(t); }
            catch (Exception e) { throw new RuntimeException(e); }
        };
    }
}

// Usage
List<String> contents = paths.stream()
    .map(ThrowingFunction.unchecked(Files::readString))
    .toList();

Collecting successes and failures

record ParseResult(List<Integer> successes, List<String> failures) {}

ParseResult parseNumbers(List<String> inputs) {
    var successes = new ArrayList<Integer>();
    var failures = new ArrayList<String>();

    for (String input : inputs) {
        try {
            successes.add(Integer.parseInt(input));
        } catch (NumberFormatException e) {
            failures.add(input);
        }
    }
    return new ParseResult(successes, failures);
}

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Common pitfalls

Pitfall	Problem	Fix
Catching Exception or Throwable broadly	Masks bugs, catches things you cannot handle	Catch specific exception types
Empty catch block	Exception silently swallowed	At minimum, log the error
Catching and rethrowing without the cause	Loses the original stack trace	throw new AppException("msg", originalException)
Using exceptions for control flow	Very slow (stack trace creation is expensive)	Use if/else, Optional, or Result
finally with return	Return in finally overrides the try-block return	Never return from finally
Logging and rethrowing	Same error logged twice (or more as it propagates)	Either log OR rethrow, not both
Not closing resources	File handles, connections leak	Use try-with-resources

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See also

• Logging -- SLF4J, Logback, and structured logging
• Optionals -- functional alternative for "no value" cases
• Functional Interfaces -- checked exceptions in lambdas
• Testing -- testing exception scenarios