Base64 in Java

How Do You Encode a String to Base64 in Java?

Use java.util.Base64.getEncoder() to obtain a Base64.Encoder instance. This class has been available since Java 8. Call encodeToString() to produce a String output, or call encode() to receive a byte[] array. The encoder uses the standard Base64 alphabet defined in RFC 4648 Section 4.

import java.util.Base64;

String original = "Hello, World!";
String encoded = Base64.getEncoder().encodeToString(original.getBytes("UTF-8"));
System.out.println(encoded); // SGVsbG8sIFdvcmxkIQ==

The getBytes("UTF-8") call converts the string to a UTF-8 byte array before encoding. Always specify the character encoding explicitly to avoid platform-dependent behavior. The encoder instance returned by getEncoder() is thread-safe and reusable. For a browser-based alternative, use the Base64 text encoder.

How Do You Decode Base64 in Java?

Use Base64.getDecoder().decode() to convert a Base64 string back to its original bytes. The decode() method accepts either a String or a byte[] and returns a byte[]. Construct a new String from the byte array with the correct character encoding.

import java.util.Base64;

byte[] decoded = Base64.getDecoder().decode("SGVsbG8sIFdvcmxkIQ==");
String result = new String(decoded, "UTF-8");
System.out.println(result); // Hello, World!

The decoder throws IllegalArgumentException if the input contains characters outside the Base64 alphabet. Validate input strings before decoding in production code. To decode Base64 directly in your browser, use the Base64 text decoder.

What Base64 Encoder Types Does Java Provide?

Java provides 3 encoder/decoder pairs through static factory methods on the Base64 class. Each pair implements a different Base64 variant defined in RFC 4648 or RFC 2045. The basic encoder handles general-purpose encoding, the URL encoder produces URL-safe output, and the MIME encoder formats output for email transport.

Each encoder has a corresponding decoder: getDecoder(), getUrlDecoder(), and getMimeDecoder(). The MIME decoder is lenient and ignores characters outside the Base64 alphabet, including line breaks. The basic and URL decoders reject non-Base64 characters with an exception. For a deeper understanding of these encoding variants, see the What is Base64 guide.

How Do You Use URL-Safe Base64 in Java?

Use Base64.getUrlEncoder() to produce URL-safe Base64 output. This encoder replaces + with - and / with _ as defined in RFC 4648 Section 5. For JWT tokens and URL parameters, chain withoutPadding() to omit trailing = characters.

import java.util.Base64;

byte[] data = "subjects?abcd".getBytes("UTF-8");

// URL-safe with padding
String withPad = Base64.getUrlEncoder().encodeToString(data);
System.out.println(withPad); // c3ViamVjdHM_YWJjZA==

// URL-safe without padding (common for JWT)
String noPad = Base64.getUrlEncoder().withoutPadding().encodeToString(data);
System.out.println(noPad); // c3ViamVjdHM_YWJjZA

The URL-safe encoder prevents conflicts when Base64 strings appear in query parameters, fragment identifiers, or file names. Decode URL-safe strings with Base64.getUrlDecoder().decode(). The URL-safe Base64 tool provides browser-based encoding with the same alphabet.

How Do You Encode a File to Base64 in Java?

Read the file into a byte array using Files.readAllBytes(), then pass the bytes to Base64.getEncoder().encodeToString(). This approach loads the entire file into memory, so it works for files that fit within the JVM heap.

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

byte[] fileBytes = Files.readAllBytes(Path.of("image.png"));
String encoded = Base64.getEncoder().encodeToString(fileBytes);

// Create a data URI for HTML embedding
String dataUri = "data:image/png;base64," + encoded;
System.out.println(dataUri);

For images, prepend the appropriate MIME type to create a data URI string. The Base64 file encoder performs the same operation in your browser without uploading the file to a server. To decode a Base64 file back to binary, reverse the process with Files.write() and Base64.getDecoder().decode().

How Do You Use Base64 with Streams in Java?

Base64.getEncoder().wrap(OutputStream) returns a Base64-encoding output stream, and Base64.getDecoder().wrap(InputStream) returns a Base64-decoding input stream. These wrappers process data incrementally, avoiding the need to load entire files into memory. Use streams for large files that exceed available heap space.

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

// Encode a file using streams (low memory usage)
try (OutputStream out = Base64.getEncoder().wrap(
        new FileOutputStream("image.b64"))) {
    Files.copy(Path.of("image.png"), out);
}

// Decode a Base64 file using streams
try (InputStream in = Base64.getDecoder().wrap(
        new FileInputStream("image.b64"))) {
    Files.copy(in, Path.of("decoded.png"));
}

The stream wrappers buffer data internally and process it in chunks. This approach handles files of any size because only a small buffer resides in memory at any time. The wrap() method is available on basic, URL-safe, and MIME encoders and decoders. Use the Base64 size calculator to estimate encoded output size before processing.

What Java Base64 Methods Are Available?

The java.util.Base64 class contains 6 static factory methods and 2 inner classes (Encoder and Decoder). The Encoder class provides 3 encoding methods plus a padding modifier and a stream wrapper. The Decoder class provides 2 decoding methods and a stream wrapper.

Base64 Static Factory Methods

Encoder Instance Methods

Decoder Instance Methods

All Encoder and Decoder instances are immutable and thread-safe. Store them as static final fields for reuse across your application. For a complete reference on the Base64 alphabet used by these methods, see the Base64 character table.

Frequently Asked Questions

Related Base64 Tools and Guides

• What is Base64? - Comprehensive guide to the Base64 encoding format
• Base64 Text Encoder - Encode text to Base64 in your browser
• URL-Safe Base64 Encoder - Encode with the URL-safe alphabet
• Base64 File Encoder - Encode files to Base64 client-side
• Base64 Character Table - Complete alphabet reference
• Base64 Size Calculator - Calculate encoded output size
• All Base64 Tools - Browse the complete toolset