Giải Mã Base64 trong Java — Hướng Dẫn getDecoder().decode()

import java.util.Base64;
import java.nio.charset.StandardCharsets;

public class DecodeCredential {
    public static void main(String[] args) {
        // Kubernetes secret value, Base64-encoded
        String encoded = "ZGItcHJvZC51cy1lYXN0LTEuYW1hem9uYXdzLmNvbTo1NDMy";

        byte[] decodedBytes = Base64.getDecoder().decode(encoded);
        String connectionString = new String(decodedBytes, StandardCharsets.UTF_8);

        System.out.println(connectionString);
        // db-prod.us-east-1.amazonaws.com:5432
    }
}

import java.util.Base64;
import java.nio.charset.StandardCharsets;

public class RoundTrip {
    public static void main(String[] args) {
        String original = "redis://cache-prod.internal:6379/session-store";

        String encoded = Base64.getEncoder().encodeToString(
            original.getBytes(StandardCharsets.UTF_8)
        );
        System.out.println(encoded);
        // cmVkaXM6Ly9jYWNoZS1wcm9kLmludGVybmFsOjYzNzkvc2Vzc2lvbi1zdG9yZQ==

        byte[] decoded = Base64.getDecoder().decode(encoded);
        String recovered = new String(decoded, StandardCharsets.UTF_8);

        System.out.println(recovered.equals(original)); // true
    }
}

import java.util.Base64;

public class DecodeToBuffer {
    public static void main(String[] args) {
        byte[] src = "eyJob3N0IjoiMTAuMC4xLjUwIiwicG9ydCI6ODQ0M30=".getBytes();
        byte[] dst = new byte[1024]; // pre-allocated

        int len = Base64.getDecoder().decode(src, dst);
        String result = new String(dst, 0, len);

        System.out.println(result);
        // {"host":"10.0.1.50","port":8443}
    }
}

import java.util.Base64;
import java.nio.ByteBuffer;
import java.util.UUID;

public class DecodeUUID {
    public static UUID fromBase64(String encoded) {
        byte[] bytes = Base64.getUrlDecoder().decode(encoded);
        ByteBuffer bb = ByteBuffer.wrap(bytes);
        return new UUID(bb.getLong(), bb.getLong());
    }

    public static void main(String[] args) {
        // Compact Base64-encoded UUID from an API response
        String encoded = "f47ac10b-58cc-4372-a567-0e02b2c3d479";
        UUID original = UUID.fromString(encoded);

        // Encode to Base64 (compact form — 22 chars vs 36)
        ByteBuffer bb = ByteBuffer.wrap(new byte[16]);
        bb.putLong(original.getMostSignificantBits());
        bb.putLong(original.getLeastSignificantBits());
        String compact = Base64.getUrlEncoder().withoutPadding()
            .encodeToString(bb.array());
        System.out.println(compact); // 9HrBC1jMQ3KlZw4CssPUeQ

        // Decode back
        UUID recovered = fromBase64(compact);
        System.out.println(recovered); // f47ac10b-58cc-4372-a567-0e02b2c3d479
    }
}

import java.util.Base64;
import java.nio.charset.StandardCharsets;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.time.Instant;

public class DecodeJsonPayload {
    record DeployEvent(String service, String region, Instant deployedAt, int replicas) {}

    public static void main(String[] args) throws Exception {
        // Base64-encoded JSON payload from a message queue
        String encoded = "eyJzZXJ2aWNlIjoicGF5bWVudC1nYXRld2F5Iiwi"
            + "cmVnaW9uIjoiZXUtd2VzdC0xIiwiZGVwbG95ZWRBdCI6"
            + "IjIwMjYtMDMtMTVUMTQ6MzA6MDBaIiwicmVwbGljYXMiOjR9";

        byte[] jsonBytes = Base64.getDecoder().decode(encoded);
        String json = new String(jsonBytes, StandardCharsets.UTF_8);
        System.out.println(json);
        // {"service":"payment-gateway","region":"eu-west-1",
        //  "deployedAt":"2026-03-15T14:30:00Z","replicas":4}

        ObjectMapper mapper = new ObjectMapper();
        mapper.findAndRegisterModules(); // picks up JavaTimeModule
        DeployEvent event = mapper.readValue(jsonBytes, DeployEvent.class);

        System.out.println(event.service());   // payment-gateway
        System.out.println(event.deployedAt()); // 2026-03-15T14:30:00Z
    }
}

import java.util.Base64;
import java.nio.charset.StandardCharsets;

public class MimeDecode {
    public static void main(String[] args) {
        // PEM certificate body — line-wrapped at 76 characters
        String pemBody = "LS0tLS1CRUdJTiBDRVJUSUZJQ0FURS0tLS0t\r\n"
            + "TUlJQm96Q0NBVWlnQXdJQkFnSUpBSXBhVDJU\r\n"
            + "aVFvZU1BMEdDU3FHU0liM0RRRU==";

        // getDecoder() would throw IllegalArgumentException here
        byte[] decoded = Base64.getMimeDecoder().decode(pemBody);
        System.out.println(new String(decoded, StandardCharsets.UTF_8));
        // -----BEGIN CERTIFICATE-----
        // MIIBozCCAUigAwIBAgIJAIpaT2T...
    }
}

import java.util.Base64;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

public class DecodeFile {
    public static void main(String[] args) {
        Path inputPath = Path.of("tls-cert.pem.b64");
        Path outputPath = Path.of("tls-cert.pem");

        try {
            String encoded = Files.readString(inputPath).strip();
            byte[] decoded = Base64.getMimeDecoder().decode(encoded);
            Files.write(outputPath, decoded);

            System.out.printf("Decoded %d bytes → %s%n", decoded.length, outputPath);
        } catch (IOException e) {
            System.err.println("File error: " + e.getMessage());
        } catch (IllegalArgumentException e) {
            System.err.println("Invalid Base64: " + e.getMessage());
        }
    }
}

import java.util.Base64;
import java.nio.charset.StandardCharsets;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.net.URI;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

public class DecodeApiResponse {
    public static void main(String[] args) {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://api.example.com/secrets/db-password"))
            .header("Authorization", "Bearer sk-prod-9f8e7d6c")
            .build();

        try {
            HttpResponse<String> response = client.send(
                request, HttpResponse.BodyHandlers.ofString());

            if (response.statusCode() != 200) {
                System.err.printf("Unexpected status: %d%n", response.statusCode());
                return;
            }

            ObjectMapper mapper = new ObjectMapper();
            JsonNode root = mapper.readTree(response.body());

            // API returns: {"name":"db-password","value":"cG9zdGdyZXM6eGs5...","version":3}
            String encodedValue = root.get("value").asText();
            byte[] decoded = Base64.getDecoder().decode(encodedValue);
            String secret = new String(decoded, StandardCharsets.UTF_8);

            System.out.println("Secret: " + secret);
            // Secret: postgres:xk9mP2qR@db-prod:5432/orders
        } catch (Exception e) {
            System.err.println("Failed to fetch secret: " + e.getMessage());
        }
    }
}

# Decode a Base64 string (Linux / macOS)
echo "eyJob3N0IjoiMTAuMC4xLjUwIiwicG9ydCI6ODQ0M30=" | base64 --decode
# {"host":"10.0.1.50","port":8443}

# Decode and pretty-print with jq
echo "eyJob3N0IjoiMTAuMC4xLjUwIiwicG9ydCI6ODQ0M30=" | base64 --decode | jq .
# {
#   "host": "10.0.1.50",
#   "port": 8443
# }

# Quick decode with jshell (JDK 9+)
echo 'System.out.println(new String(java.util.Base64.getDecoder().decode("c2VydmVyLWNvbmZpZw==")))' | jshell -
# server-config

# macOS uses -D instead of --decode
echo "c2VydmVyLWNvbmZpZw==" | base64 -D

<!-- pom.xml -->
<dependency>
    <groupId>commons-codec</groupId>
    <artifactId>commons-codec</artifactId>
    <version>1.17.0</version>
</dependency>

import org.apache.commons.codec.binary.Base64;

public class CommonsCodecDecode {
    public static void main(String[] args) {
        // Commons Codec is more lenient — handles whitespace and line breaks
        String encoded = "eyJob3N0IjoiMTAuMC4xLjUw\nIiwicG9ydCI6ODQ0M30=";
        byte[] decoded = Base64.decodeBase64(encoded);

        System.out.println(new String(decoded));
        // {"host":"10.0.1.50","port":8443}
    }
}

import java.util.Base64;
import java.io.*;
import java.nio.file.*;

public class StreamDecode {
    public static void main(String[] args) throws IOException {
        Path src = Path.of("database-dump.sql.b64");
        Path dst = Path.of("database-dump.sql");

        try (InputStream in = Base64.getMimeDecoder().wrap(
                 new BufferedInputStream(Files.newInputStream(src)));
             OutputStream out = new BufferedOutputStream(Files.newOutputStream(dst))) {

            byte[] buffer = new byte[8192];
            int bytesRead;
            long total = 0;
            while ((bytesRead = in.read(buffer)) != -1) {
                out.write(buffer, 0, bytesRead);
                total += bytesRead;
            }
            System.out.printf("Decoded %d bytes → %s%n", total, dst);
        }
    }
}

import java.util.Base64;
import java.io.*;
import java.nio.file.*;

public class StreamDecodeSimple {
    public static void main(String[] args) throws IOException {
        try (InputStream in = Base64.getMimeDecoder().wrap(
                 new BufferedInputStream(Files.newInputStream(Path.of("backup.tar.b64"))));
             OutputStream out = Files.newOutputStream(Path.of("backup.tar"))) {
            in.transferTo(out); // Java 9+
        }
    }
}

import java.util.Base64;
import java.nio.charset.StandardCharsets;

public class JWTInspect {
    public static String decodeJwtPayload(String token) {
        String[] parts = token.split("\\.");
        if (parts.length != 3) {
            throw new IllegalArgumentException(
                "Invalid JWT: expected 3 segments, got " + parts.length);
        }
        // JWT uses URL-safe Base64 without padding
        byte[] payload = Base64.getUrlDecoder().decode(parts[1]);
        return new String(payload, StandardCharsets.UTF_8);
    }

    public static void main(String[] args) {
        String token = "eyJhbGciOiJSUzI1NiJ9"
            + ".eyJzdWIiOiJ1c3ItNjcyIiwiaXNzIjoiYXV0aC5leGFtcGxlLmNvbSIs"
            + "ImV4cCI6MTc0MTk1NjgwMCwicm9sZXMiOlsiYWRtaW4iLCJiaWxsaW5nIl19"
            + ".SIGNATURE_PLACEHOLDER";

        String payload = decodeJwtPayload(token);
        System.out.println(payload);
        // {"sub":"usr-672","iss":"auth.example.com",
        //  "exp":1741956800,"roles":["admin","billing"]}
    }
}