JWT Decoding

/**
 * Decode a given Json Web Token.
 * Note that this method doesn't verify the token's signature!
 * Use it only if you trust the token or if you have already verified it.
 * @param token with jwt format as string
 * @return a decoded JWT
 * @throws JWTDecodeException if any part of the token contained an invalid jwt or JSON format of each of the jwt parts
 */
public static DecodedJWT decode(String token) throws JWTDecodeException;

import com.auth0.jwt.JWT;
import com.auth0.jwt.interfaces.DecodedJWT;
import com.auth0.jwt.exceptions.JWTDecodeException;

try {
    String token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c";
    
    DecodedJWT jwt = JWT.decode(token);
    
    // Access header information
    String algorithm = jwt.getAlgorithm();
    String keyId = jwt.getKeyId();
    
    // Access payload claims
    String subject = jwt.getSubject();
    String issuer = jwt.getIssuer();
    Date expiration = jwt.getExpiresAt();
    
    System.out.println("Algorithm: " + algorithm);
    System.out.println("Subject: " + subject);
    
} catch (JWTDecodeException exception) {
    System.err.println("Invalid JWT format: " + exception.getMessage());
}

/**
 * Decode a given Json Web Token using a JWT instance.
 * Note that this method doesn't verify the token's signature!
 * Use it only if you trust the token or if you have already verified it.
 * @param token with jwt format as string
 * @return a decoded JWT
 * @throws JWTDecodeException if any part of the token contained an invalid jwt or JSON format of each of the jwt parts
 */
public DecodedJWT decodeJwt(String token) throws JWTDecodeException;

import com.auth0.jwt.JWT;
import com.auth0.jwt.interfaces.DecodedJWT;
import com.auth0.jwt.exceptions.JWTDecodeException;

// Create reusable JWT instance for high-throughput decoding
JWT jwtDecoder = new JWT();

// Decode multiple tokens efficiently
for (String token : tokenList) {
    try {
        DecodedJWT jwt = jwtDecoder.decodeJwt(token);
        String subject = jwt.getSubject();
        processToken(subject);
    } catch (JWTDecodeException exception) {
        System.err.println("Invalid JWT: " + exception.getMessage());
    }
}

**Usage Example:**

```java
import com.auth0.jwt.JWT;
import com.auth0.jwt.interfaces.DecodedJWT;

// Create reusable decoder instance
JWT jwtDecoder = new JWT();

// Decode multiple tokens efficiently
String[] tokens = {
    "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...",
    "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9...",
    "eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9..."
};

for (String token : tokens) {
    try {
        DecodedJWT jwt = jwtDecoder.decodeJwt(token);
        System.out.println("Token subject: " + jwt.getSubject());
    } catch (JWTDecodeException e) {
        System.err.println("Failed to decode token: " + e.getMessage());
    }
}

/**
 * Getter for the Token used to create this JWT instance.
 * @return the Token used to create this JWT instance
 */
String getToken();

/**
 * Getter for the Header contained in this JWT as a Base64 encoded String.
 * @return the Header contained in this JWT as a Base64 encoded String
 */
String getHeader();

/**
 * Getter for the Payload contained in this JWT as a Base64 encoded String.
 * @return the Payload contained in this JWT as a Base64 encoded String
 */
String getPayload();

/**
 * Getter for the Signature contained in this JWT as a Base64 encoded String.
 * @return the Signature contained in this JWT as a Base64 encoded String
 */
String getSignature();

DecodedJWT jwt = JWT.decode(token);

// Access raw components
String originalToken = jwt.getToken();
String headerB64 = jwt.getHeader();
String payloadB64 = jwt.getPayload();
String signatureB64 = jwt.getSignature();

System.out.println("Original token: " + originalToken);
System.out.println("Header (Base64): " + headerB64);
System.out.println("Payload (Base64): " + payloadB64);
System.out.println("Signature (Base64): " + signatureB64);

// Reconstruct token manually if needed
String reconstructed = headerB64 + "." + payloadB64 + "." + signatureB64;
assert originalToken.equals(reconstructed);

/**
 * Getter for the Algorithm "alg" claim contained in the Header.
 * @return the Algorithm defined or null if it's not defined in the Header
 */
String getAlgorithm();

/**
 * Getter for the Type "typ" claim contained in the Header.
 * @return the Type defined or null if it's not defined in the Header
 */
String getType();

/**
 * Getter for the Content Type "cty" claim contained in the Header.
 * @return the Content Type defined or null if it's not defined in the Header
 */
String getContentType();

/**
 * Getter for the Key Id "kid" claim contained in the Header.
 * @return the Key Id defined or null if it's not defined in the Header
 */
String getKeyId();

/**
 * Get a custom Claim given its name from the Header.
 * @param name the name of the Claim to retrieve
 * @return a valid Claim instance if the Claim was found or a NullClaim if not
 */
Claim getHeaderClaim(String name);

DecodedJWT jwt = JWT.decode(token);

// Standard header claims
String algorithm = jwt.getAlgorithm();    // e.g., "HS256", "RS256"
String type = jwt.getType();              // typically "JWT"
String contentType = jwt.getContentType(); // e.g., "application/json"
String keyId = jwt.getKeyId();            // key identifier for verification

System.out.println("Algorithm: " + algorithm);
System.out.println("Type: " + type);

// Custom header claims
Claim customClaim = jwt.getHeaderClaim("custom");
if (!customClaim.isMissing()) {
    String customValue = customClaim.asString();
    System.out.println("Custom header claim: " + customValue);
}

/**
 * Getter for the Issuer "iss" claim contained in the Payload.
 * @return the Issuer value or null if it's not defined in the Payload
 */
String getIssuer();

/**
 * Getter for the Subject "sub" claim contained in the Payload.
 * @return the Subject value or null if it's not defined in the Payload
 */
String getSubject();

/**
 * Getter for the Audience "aud" claim contained in the Payload.
 * @return the Audience value or an empty list if it's not defined in the Payload
 */
List<String> getAudience();

/**
 * Getter for the Expires At "exp" claim contained in the Payload.
 * @return the Expires At value or null if it's not defined in the Payload
 */
Date getExpiresAt();

/**
 * Getter for the Expires At "exp" claim contained in the Payload as an Instant.
 * @return the Expires At value or null if it's not defined in the Payload
 */
Instant getExpiresAtAsInstant();

/**
 * Getter for the Not Before "nbf" claim contained in the Payload.
 * @return the Not Before value or null if it's not defined in the Payload
 */
Date getNotBefore();

/**
 * Getter for the Not Before "nbf" claim contained in the Payload as an Instant.
 * @return the Not Before value or null if it's not defined in the Payload
 */
Instant getNotBeforeAsInstant();

/**
 * Getter for the Issued At "iat" claim contained in the Payload.
 * @return the Issued At value or null if it's not defined in the Payload
 */
Date getIssuedAt();

/**
 * Getter for the Issued At "iat" claim contained in the Payload as an Instant.
 * @return the Issued At value or null if it's not defined in the Payload
 */
Instant getIssuedAtAsInstant();

/**
 * Getter for the JWT ID "jti" claim contained in the Payload.
 * @return the JWT ID value or null if it's not defined in the Payload
 */
String getId();

/**
 * Get a custom Claim given its name from the Payload.
 * @param name the name of the Claim to retrieve
 * @return a valid Claim instance if the Claim was found or a NullClaim if not
 */
Claim getClaim(String name);

/**
 * Get all Claims contained in the Payload.
 * @return a Map containing all the custom Claims defined in the token
 */
Map<String, Claim> getClaims();

DecodedJWT jwt = JWT.decode(token);

// Standard registered claims
String issuer = jwt.getIssuer();           // "iss" claim
String subject = jwt.getSubject();         // "sub" claim
List<String> audience = jwt.getAudience(); // "aud" claim
Date expiresAt = jwt.getExpiresAt();       // "exp" claim
Date notBefore = jwt.getNotBefore();       // "nbf" claim
Date issuedAt = jwt.getIssuedAt();         // "iat" claim
String jwtId = jwt.getId();                // "jti" claim

System.out.println("Issuer: " + issuer);
System.out.println("Subject: " + subject);
System.out.println("Expires at: " + expiresAt);

// Use Instant for modern date handling
Instant expiresAtInstant = jwt.getExpiresAtAsInstant();
Instant notBeforeInstant = jwt.getNotBeforeAsInstant();
Instant issuedAtInstant = jwt.getIssuedAtAsInstant();

// Custom claims
Claim roleClaim = jwt.getClaim("role");
if (!roleClaim.isMissing()) {
    String role = roleClaim.asString();
    System.out.println("Role: " + role);
}

// All claims
Map<String, Claim> allClaims = jwt.getClaims();
for (Map.Entry<String, Claim> entry : allClaims.entrySet()) {
    String claimName = entry.getKey();
    Claim claimValue = entry.getValue();
    if (!claimValue.isMissing()) {
        System.out.println(claimName + ": " + claimValue.asString());
    }
}

// ⚠️ SECURITY WARNING: This does NOT verify the signature!
DecodedJWT jwt = JWT.decode(untrustedToken);

// ✅ SECURE: This verifies the signature
JWTVerifier verifier = JWT.require(algorithm).build();
DecodedJWT verifiedJwt = verifier.verify(untrustedToken);

/**
 * JWT decoder interface providing access to token components and claims
 */
public interface DecodedJWT extends Payload, Header {
    String getToken();
    String getHeader();
    String getPayload();
    String getSignature();
}

/**
 * JWT header interface providing access to header claims
 */
public interface Header {
    String getAlgorithm();
    String getType();
    String getContentType();
    String getKeyId();
    Claim getHeaderClaim(String name);
}

/**
 * JWT payload interface providing access to payload claims
 */
public interface Payload {
    String getIssuer();
    String getSubject();
    List<String> getAudience();
    Date getExpiresAt();
    Instant getExpiresAtAsInstant();
    Date getNotBefore();
    Instant getNotBeforeAsInstant();
    Date getIssuedAt();
    Instant getIssuedAtAsInstant();
    String getId();
    Claim getClaim(String name);
    Map<String, Claim> getClaims();
}