java 41 lines · 6 steps

Java try-with-resources and AutoCloseable

How implementing AutoCloseable lets try-with-resources clean up your objects automatically, even when code throws.

Explained by highlit
Intermediate try-with-resources autocloseable resource-management cleanup exception-safety
1import java.io.BufferedReader;
2import java.io.FileReader;
3import java.io.IOException;
4 
5public class ResourceManagement {
6 
7 // A custom AutoCloseable resource whose close() is invoked automatically.
8 static class DatabaseConnection implements AutoCloseable {
9 private final String name;
10 
11 DatabaseConnection(String name) {
12 this.name = name;
13 System.out.println("Opening connection: " + name);
14 }
15 
16 void query(String sql) {
17 System.out.println("[" + name + "] executing: " + sql);
18 }
19 
20 @Override
21 public void close() {
22 System.out.println("Closing connection: " + name);
23 }
24 }
25 
26 // Single resource: close() runs whether the block completes or throws.
27 String readFirstLine(String path) throws IOException {
28 try (BufferedReader reader = new BufferedReader(new FileReader(path))) {
29 return reader.readLine();
30 }
31 }
32 
33 // Multiple resources are closed in reverse order of declaration.
34 void runTransaction() {
35 try (DatabaseConnection primary = new DatabaseConnection("primary");
36 DatabaseConnection replica = new DatabaseConnection("replica")) {
37 primary.query("INSERT INTO orders VALUES (1)");
38 replica.query("SELECT * FROM orders");
39 }
40 }
41}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Any class implementing AutoCloseable can participate in try-with-resources, not just built-in IO types.
  2. 2close() is guaranteed to run when the try block exits, whether it returns normally or throws.
  3. 3Multiple resources in one try header are closed in reverse order of declaration, mirroring safe teardown.

Related explainers

java 
public interface GitHubClient {
 
    @GetExchange("/users/{username}")
    GitHubUser getUser(@PathVariable String username);

Declarative HTTP clients in Spring

http-client declarative-api proxy
Intermediate 8 steps
java 
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

Building a trie for autocomplete in Java

trie prefix-tree recursion
Intermediate 8 steps
java 
@RestController
@RequestMapping("/api/products")
public class ProductSearchController {
 

Binding collection query params in Spring

rest-api query-parameters dependency-injection
Intermediate 6 steps
java 
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Map;
 

Evaluating math expressions with two stacks

stacks parsing operator-precedence
Intermediate 9 steps
java 
@Entity
@Table(name = "orders")
@SQLDelete(sql = "UPDATE orders SET deleted = true, deleted_at = now() WHERE id = ?")
@Where(clause = "deleted = false")

Soft deletes with Hibernate in Spring

soft-delete jpa hibernate
Intermediate 9 steps
javascript 
import { useState, useEffect, useCallback } from 'react';
 
function getColumnCount(width) {
  if (width < 640) return 1;

A responsive column hook in React

custom-hooks debouncing responsive-design
Intermediate 7 steps

Share this explainer

Here's the card — post it anywhere.

Java try-with-resources and AutoCloseable — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code