Extend post-processing for type field

Author: malczuuu

README.md

@@ -159,7 +159,7 @@ declaratively map exception fields to a `Problem`.
 To extract values from target exception, it's possible to use placeholders for interpolation.
 
 - `{message}` - the exact `getMessage()` result from your exception,
-- `{traceId}` - the `context.getTraceId()` result for tracking error response with the actual request. The `context` is
+- `{context.traceId}` - the `context.getTraceId()` result for tracking error response with the actual request. The `context` is
   something that is build in `@RestControllerAdvice`s and it contains processing metadata. Currently only `traceId` is
   supported,
 - `{fieldName}` - any field name declared in exceptions and its superclasses (scanned from current class to its most
@@ -184,7 +184,7 @@ To extract values from target exception, it's possible to use placeholders for i
     title = "Invalid Request",
     status = 400,
     detail = "{message}: {fieldName}",
-    instance = "https://example.org/instances/{traceId}",
+    instance = "https://example.org/instances/{context.traceId}",
     extensions = {"userId", "fieldName"})
 public class ExampleException extends RuntimeException {
 
@@ -237,7 +237,7 @@ public class MaxUploadSizeExceededResolver implements ProblemResolver {
 ```
 
 `ProblemResolver` implementations return a `ProblemBuilder` for flexibility in constructing the final `Problem` object.
-It's a convenience for additional `Problem` processing (such as `instance-override` feature).
+It's a convenience method for further extending `Problem` object by processing downstream.
 
 #### Custom `@RestControllerAdvice`
 
@@ -257,36 +257,40 @@ If you want your advice to override the ones provided by this library, use a sma
 | `ProblemException`                  | `Ordered.LOWEST_PRECEDENCE - 10` |
 | `Exception`                         | `Ordered.LOWEST_PRECEDENCE`      |
 
-While implementing custom `@ControllerAdvice`, enforcing `instance-override` feature must be performed manually. Value
-of `"instance"` field will come from request attributes and must be overwritten if not `null`.
+While implementing custom `@ControllerAdvice`, don't forget of calling `ProblemPostProcessor` manually, before returning
+`Problem` object. 
 
 ```java
 @Order(Ordered.LOWEST_PRECEDENCE - 20)
 @Component
 @RestControllerAdvice
 public class ExampleExceptionAdvice {
 
+  private final ProblemPostProcessor problemPostProcessor;
+
+  // constructor
+
   @ExceptionHandler(ExampleException.class)
   public ResponseEntity<Problem> handleExampleException(ExampleException ex, WebRequest request) {
-    ProblemBuilder =
+    ProblemContext context = (ProblemContext) request.getAttribute(PROBLEM_CONTEXT, SCOPE_REQUEST);
+    if (context == null) {
+      context = ProblemContext.empty();
+    }
+
+    HttpHeaders headers = new HttpHeaders();
+    headers.setContentType(MediaType.APPLICATION_PROBLEM_JSON);
+
+    Problem problem =
         Problem.builder()
             .type("https://example.org/errors/invalid-request")
             .title("Invalid Request")
             .status(400)
             .detail(ex.getMessage())
             .instance("https://example.org/instances/" + context.getTraceId())
             .extension("userId", e.getUserId())
-            .extension("fieldName", e.getFieldName());
-
-    Object instanceOverride = request.getAttribute(TracingSupport.INSTANCE_OVERRIDE, SCOPE_REQUEST);
-    if (instanceOverride != null) {
-      builder = builder.instance(instanceOverride.toString());
-    }
-
-    Problem problem = builder.build();
-
-    HttpHeaders headers = new HttpHeaders();
-    headers.setContentType(MediaType.APPLICATION_PROBLEM_JSON);
+            .extension("fieldName", e.getFieldName())
+            .build();
+    problem = problemPostProcessor.process(context, problem);
 
     HttpStatus status = ProblemSupport.resolveStatus(problem.getStatus());
 
@@ -538,24 +542,62 @@ Library can be configured with following properties.
 
 ### `problem4j.detail-format`
 
-Property that specifies how exception handling imported with this module should print `"detail"` field of `Problem`
-model (`lowercase`, **`capitalized` - default**, `uppercase`). Useful for keeping the same style of errors coming from
-library and your application.
+Property that specifies how exception handling imported with this module should print the `"detail"` field of the
+`Problem` model (`lowercase`, **`capitalized` - default**, `uppercase`). Useful for keeping a consistent style between
+errors generated by the library and those from your application.
 
 ### `problem4j.tracing-header-name`
 
 Property that specifies the name of the HTTP header used for tracing requests. If set, the trace identifier from this
-header can be injected into the `Problem` response, for example into the`"instance"` field when combined with
-[`problem4j.instance-override`](#problem4jinstance-override). Defaults to `null` (disabled).
+header is extracted and made available within the request context (`ProblemContext`). This value can be referenced in
+other configuration properties using the `{context.traceId}` placeholder. Defaults to `null` (disabled).
+
+### `problem4j.type-override`
+
+This property allow overriding the `"type"` field of `Problem` responses with custom templates at the environment level.
+This is useful when the final URIs are not known at development time or may vary depending on deployment, enabling them
+to resolve to proper HTTP links dynamically.
+
+Property that defines a template for overriding the `"type"` field in `Problem` responses. The value may contain special
+placeholders that will be replaced at runtime:
+
+- `{problem.type}` — the original problem’s `"type"` value
+- `{context.traceId}` — the current request’s trace identifier (if available)
+
+Defaults to `null` (no override applied). This can be used to unify or enrich problem type URIs across the application.
+
+> For example, if setting `type` in your exception to `problems/validation` and:
+> 
+> ```properties
+> problem4j.type-override=https://errors.example.com/{problem.type}
+> ```
+> 
+> the post processor will change `"type"` value to `"https://errors.example.com/problems/validation`.
 
 ### `problem4j.instance-override`
 
-Property that defines a template for overriding the `"instance"` field in `Problem` responses.The value may contain the
-special placeholder `{traceId}`, which will be replaced at runtime with the trace identifier from the current request (
-see [`problem4j.tracing-header-name`](#problem4jtracing-header-name)). Defaults to `null` (no override applied).
+This property allow overriding the `"instance"` field of `Problem` responses with custom templates at the environment
+level. This is useful when the final URIs are not known at development time or may vary depending on deployment,
+enabling them to resolve to proper HTTP links dynamically.
+
+Property that defines a template for overriding the `"instance"` field in `Problem` responses. The value may contain
+special placeholders that will be replaced at runtime:
+
+- `{problem.instance}` — the original problem’s `"instance"` value
+- `{context.traceId}` — the current request’s trace identifier (if available)
+
+Defaults to `null` (no override applied). This is useful for controlling how problem instances are represented in your
+API responses, even without tracing enabled.
 
-For example, by assigning `problem4j.instance-override=/error-instances/{traceId}`, with tracing enabled, each `Problem`
-response will have `"instance"` field matching to that format (e.g. `"/error-instances/WQ1tbs12rtSD"`).
+> For example if not assigning `instance` at all in your exceptions and: 
+> 
+> ```properties
+> problem4j.instance-override=/errors/{context.traceId}
+> ```
+> 
+> the post processor will change `"instance"` value to `"/errors/WQ1tbs12rtSD"`.
+> 
+> For using `{problem.instance}` placeholder, the `instance` field will behave similarly to `type-override`.
 
 ### `problem4j.resolver-caching.enabled`
 

problem4j-spring-web/src/main/java/io/github/malczuuu/problem4j/spring/web/ProblemConfiguration.java

@@ -5,6 +5,8 @@
 import io.github.malczuuu.problem4j.spring.web.annotation.ProblemMappingProcessor;
 import io.github.malczuuu.problem4j.spring.web.format.DefaultProblemFormat;
 import io.github.malczuuu.problem4j.spring.web.format.ProblemFormat;
+import io.github.malczuuu.problem4j.spring.web.processor.OverridingProblemPostProcessor;
+import io.github.malczuuu.problem4j.spring.web.processor.ProblemPostProcessor;
 import io.github.malczuuu.problem4j.spring.web.resolver.ProblemResolver;
 import io.github.malczuuu.problem4j.spring.web.resolver.ProblemResolverConfiguration;
 import java.util.List;
@@ -43,6 +45,33 @@ public ProblemFormat problemFormat(ProblemProperties properties) {
     return new DefaultProblemFormat(properties.getDetailFormat());
   }
 
+  /**
+   * Provides a {@link ProblemPostProcessor} that applies post-processing rules to {@code Problem}
+   * instances before they are returned in HTTP responses.
+   *
+   * <p>The default implementation, {@link OverridingProblemPostProcessor}, supports configurable
+   * overrides for problem fields such as {@code type} and {@code instance}, based on the properties
+   * defined in {@link ProblemProperties}. These overrides may include runtime placeholders such as:
+   *
+   * <ul>
+   *   <li>{@code {problem.type}} — replaced with the original problem’s type URI
+   *   <li>{@code {problem.instance}} — replaced with the original problem’s instance URI
+   *   <li>{@code {context.traceId}} — replaced with the current trace identifier, if available
+   * </ul>
+   *
+   * <p>This allows enriching or normalizing problem responses without modifying the original
+   * exception mapping logic.
+   *
+   * @param properties the configuration properties containing override templates and settings
+   * @return a new {@link OverridingProblemPostProcessor} instance
+   * @see io.github.malczuuu.problem4j.core.Problem
+   */
+  @ConditionalOnMissingBean(ProblemPostProcessor.class)
+  @Bean
+  public ProblemPostProcessor problemPostProcessor(ProblemProperties properties) {
+    return new OverridingProblemPostProcessor(properties);
+  }
+
   /**
    * Provides a {@link ProblemResolverStore} that aggregates all {@link ProblemResolver}
    * implementations.

problem4j-spring-web/src/main/java/io/github/malczuuu/problem4j/spring/web/ProblemProperties.java

@@ -1,5 +1,7 @@
 package io.github.malczuuu.problem4j.spring.web;
 
+import io.github.malczuuu.problem4j.spring.web.context.ProblemContextSettings;
+import io.github.malczuuu.problem4j.spring.web.processor.PostProcessorSettings;
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.boot.context.properties.bind.DefaultValue;
 
@@ -9,10 +11,11 @@
  * <p>These properties can be set under the {@code problem4j.*} prefix.
  */
 @ConfigurationProperties(prefix = "problem4j")
-public class ProblemProperties {
+public class ProblemProperties implements ProblemContextSettings, PostProcessorSettings {
 
   private final String detailFormat;
   private final String tracingHeaderName;
+  private final String typeOverride;
   private final String instanceOverride;
 
   private final ResolverCaching resolverCaching;
@@ -23,18 +26,20 @@ public class ProblemProperties {
    * @param detailFormat format for the "detail" field (one of {@link DetailFormat#LOWERCASE},
    *     {@link DetailFormat#CAPITALIZED}, {@link DetailFormat#UPPERCASE})
    * @param tracingHeaderName name of the HTTP header carrying a trace ID (nullable)
-   * @param instanceOverride template for overriding the "instance" field; may contain "{traceId}"
-   *     placeholder (nullable)
+   * @param instanceOverride template for overriding the "instance" field; may contain
+   *     "{context.traceId}" placeholder (nullable)
    * @param resolverCaching caching for resolver lookups ({@link CachingProblemResolverStore});
    *     defaults to {@link ResolverCaching#createDefault()}
    */
   public ProblemProperties(
       @DefaultValue(DetailFormat.CAPITALIZED) String detailFormat,
       String tracingHeaderName,
+      String typeOverride,
       String instanceOverride,
       ResolverCaching resolverCaching) {
     this.detailFormat = detailFormat;
     this.tracingHeaderName = tracingHeaderName;
+    this.typeOverride = typeOverride;
     this.instanceOverride = instanceOverride;
     this.resolverCaching =
         resolverCaching != null ? resolverCaching : ResolverCaching.createDefault();
@@ -60,24 +65,56 @@ public String getDetailFormat() {
    *
    * @return the tracing header name, or {@code null} if not set
    */
+  @Override
   public String getTracingHeaderName() {
     return tracingHeaderName;
   }
 
+  /**
+   * Returns the configured type override.
+   *
+   * <p>This value defines a fixed or templated {@code type} URI to be used for all problems
+   * processed by the post-processor, replacing the original problem type if present. The value may
+   * include special placeholders that will be dynamically replaced at runtime:
+   *
+   * <ul>
+   *   <li>{@code {problem.type}} — replaced with the original problem’s type URI
+   *   <li>{@code {context.traceId}} — replaced with the current trace identifier from the {@code
+   *       ProblemContext}
+   * </ul>
+   *
+   * <p>This allows flexible configuration of problem types depending on context or trace
+   * information. If no override is configured, this method may return {@code null}, and the
+   * original problem type will be preserved.
+   *
+   * @return the configured type override string, or {@code null} if not set
+   * @see io.github.malczuuu.problem4j.spring.web.context.ProblemContext
+   */
+  @Override
+  public String getTypeOverride() {
+    return typeOverride;
+  }
+
   /**
    * Returns the configured instance override.
    *
-   * <p>This value may contain the special placeholder {@code {traceId}}, which will be replaced at
-   * runtime with the current trace identifier from the {@link
-   * io.github.malczuuu.problem4j.spring.web.context.ProblemContext}. If no override is configured,
-   * this method may return {@code null}.
+   * <p>This value may contain special placeholders that will be replaced at runtime with contextual
+   * or problem-specific data:
+   *
+   * <ul>
+   *   <li>{@code {problem.instance}} — replaced with the original problem’s instance URI
+   *   <li>{@code {context.traceId}} — replaced with the current trace identifier from the {@code
+   *       ProblemContext}
+   * </ul>
    *
-   * <p>This is useful if {@code instance} field will not be known while throwing {@code
-   * ProblemException} (or {@code @ProblemMapping}-annotated exception). Setting this configuration,
-   * along with {@link #tracingHeaderName} will enable this feature.
+   * <p>This is useful if the {@code instance} field cannot be determined when throwing a {@code
+   * ProblemException} (or an exception annotated with {@code @ProblemMapping}). Setting this
+   * configuration, along with {@link #tracingHeaderName}, enables this feature.
    *
    * @return the configured instance override string, or {@code null} if not set
+   * @see io.github.malczuuu.problem4j.spring.web.context.ProblemContext
    */
+  @Override
   public String getInstanceOverride() {
     return instanceOverride;
   }

problem4j-spring-web/src/main/java/io/github/malczuuu/problem4j/spring/web/annotation/DefaultProblemMappingProcessor.java

@@ -21,7 +21,8 @@
  *         <li>Interpolate placeholders of the form {@code {name}}:
  *             <ul>
  *               <li>{@code {message}} -> {@link Throwable#getMessage()}
- *               <li>{@code {traceId}} -> {@link ProblemContext#getTraceId()} (special shorthand)
+ *               <li>{@code {context.traceId}} -> {@link ProblemContext#getTraceId()} (special
+ *                   shorthand)
  *               <li>{@code {fieldName}} -> any field in the exception class hierarchy
  *             </ul>
  *         <li>Ignore placeholders that resolve to null or empty string.
@@ -178,8 +179,8 @@ private void applyExtensionsOnBuilder(
   }
 
   /**
-   * Interpolate placeholders of form {name}. Special forms: - {message} - {context.key} - {traceId}
-   * (shorthand for {context.traceId}) - other names: looks for instance field.
+   * Interpolate placeholders of form {name}. Special forms: - {message} - {context.key} -
+   * {context.traceId} (shorthand for {context.traceId}) - other names: looks for instance field.
    *
    * <p>If a placeholder resolves to null - it's replaced by empty string.
    */

problem4j-spring-web/src/main/java/io/github/malczuuu/problem4j/spring/web/annotation/ProblemMapping.java

@@ -17,7 +17,7 @@
  *
  * <ul>
  *   <li>{@code {message}} -> the exception's {@link Throwable#getMessage()}
- *   <li>{@code {traceId}} -> the traceId from {@code ProblemContext#getTraceId()} (special
+ *   <li>{@code {context.traceId}} -> the traceId from {@code ProblemContext#getTraceId()} (special
  *       shorthand)
  *   <li>{@code {fieldName}} -> value of any field (private or public) in the exception class
  *       hierarchy
@@ -52,7 +52,7 @@
  *     type = "https://example.org/errors/validation",
  *     title = "Validation Failed",
  *     status = 400,
- *     detail = "Invalid input for user {userId}, trace {traceId}",
+ *     detail = "Invalid input for user {userId}, trace {context.traceId}",
  *     extensions = {"userId", "fieldName"}
  * )
  * public class ValidationException extends RuntimeException {
@@ -85,7 +85,7 @@
    *
    * <ul>
    *   <li>{@code {message}} -> exception message
-   *   <li>{@code {traceId}} -> trace ID from {@code ProblemContext}
+   *   <li>{@code {context.traceId}} -> trace ID from {@code ProblemContext}
    *   <li>{@code {fieldName}} -> value of any field in the exception class hierarchy
    * </ul>
    *

problem4j-spring-web/src/main/java/io/github/malczuuu/problem4j/spring/web/annotation/ProblemMappingProcessor.java

@@ -32,7 +32,7 @@ public interface ProblemMappingProcessor {
   Pattern PLACEHOLDER = Pattern.compile("\\{([^}]+)}");
 
   String MESSAGE_LABEL = "message";
-  String TRACE_ID_LABEL = "traceId";
+  String TRACE_ID_LABEL = "context.traceId";
 
   /**
    * Convert {@link Throwable} -> {@link ProblemBuilder} according to its {@link ProblemMapping}

problem4j-spring-web/src/main/java/io/github/malczuuu/problem4j/spring/web/context/ContextSupport.java

@@ -0,0 +1,32 @@
+package io.github.malczuuu.problem4j.spring.web.context;
+
+import java.util.UUID;
+
+/**
+ * Utility class providing constants and helper methods for tracing support within the Problem4J.
+ */
+public final class ContextSupport {
+
+  /** Request attribute key used to store a trace identifier. */
+  public static final String TRACE_ID =
+      "io.github.malczuuu.problem4j.spring.web.context.ProblemContext.traceId";
+
+  /**
+   * Request attribute key used to store the {@link ProblemContext} object associated with the
+   * current request. It allows sharing contextual information (such as trace identifiers or
+   * additional diagnostic data) between components involved in problem handling.
+   */
+  public static final String PROBLEM_CONTEXT =
+      "io.github.malczuuu.problem4j.spring.web.context.ProblemContext";
+
+  /**
+   * Generates a random trace identifier in {@code urn:uuid:<uuid>} format.
+   *
+   * @return generated trace identifier
+   */
+  public static String getRandomTraceId() {
+    return "urn:uuid:" + UUID.randomUUID();
+  }
+
+  private ContextSupport() {}
+}