Add response body matchers (#37)

* add ResponseBodyMatchers used in EMVA and AICHNER

* make json mapper private

* fix reviewed issues

* fix style violations

* fix reviewed issues

* fix reviewed issues

Author: Piiit

src/main/java/it/aboutbits/springboot/testing/web/response/ResponseBodyMatchers.java

@@ -0,0 +1,98 @@
+package it.aboutbits.springboot.testing.web.response;
+
+import com.jayway.jsonpath.JsonPath;
+import org.jspecify.annotations.NullMarked;
+import org.springframework.test.web.servlet.ResultMatcher;
+import tools.jackson.databind.json.JsonMapper;
+
+import java.math.BigDecimal;
+import java.time.LocalDateTime;
+import java.time.OffsetDateTime;
+import java.time.temporal.ChronoUnit;
+import java.util.ArrayList;
+import java.util.Arrays;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
+import static org.assertj.core.api.Assertions.fail;
+
+@NullMarked
+public final class ResponseBodyMatchers {
+    private static final JsonMapper JSON_MAPPER = new JsonMapper();
+    private final String jsonPath;
+
+    private String[] fieldNamesToIgnore = new String[0];
+    private String[] optionalFieldNamesToIgnore = new String[0];
+    private boolean ignoreAuditFields = false;
+
+    private ResponseBodyMatchers(String jsonPath) {
+        this.jsonPath = jsonPath;
+    }
+
+    public static ResponseBodyMatchers responseBody() {
+        return new ResponseBodyMatchers("$");
+    }
+
+    public static ResponseBodyMatchers responseBodyAt(String expression) {
+        return new ResponseBodyMatchers(expression);
+    }
+
+    public ResponseBodyMatchers ignoringOptionalFields(String... optionalFieldNamesToIgnore) {
+        this.optionalFieldNamesToIgnore = optionalFieldNamesToIgnore;
+        return this;
+    }
+
+    public ResponseBodyMatchers ignoringFields(String... fieldNamesToIgnore) {
+        this.fieldNamesToIgnore = fieldNamesToIgnore;
+        return this;
+    }
+
+    public ResponseBodyMatchers ignoringAuditFields() {
+        this.ignoreAuditFields = true;
+        return this;
+    }
+
+    public <T> ResultMatcher containsObjectAsJson(
+            Object expectedObject,
+            Class<T> targetClass
+    ) {
+        return mvcResult -> {
+            try {
+                var json = mvcResult.getResponse().getContentAsString();
+                var extractedValue = JsonPath.read(json, jsonPath);
+                T actualObject = JSON_MAPPER.convertValue(extractedValue, targetClass);
+
+                var ignoredFields = new ArrayList<>(Arrays.asList(fieldNamesToIgnore));
+                ignoredFields.addAll(Arrays.asList(optionalFieldNamesToIgnore));
+                if (ignoreAuditFields) {
+                    ignoredFields.addAll(Arrays.asList("createdAt", "createdBy", "updatedAt", "updatedBy"));
+                }
+
+                assertThat(actualObject)
+                        .usingComparatorForType(BigDecimal::compareTo, BigDecimal.class)
+                        .usingComparatorForType(ResponseBodyMatchers::compareTemporalByDelta, OffsetDateTime.class)
+                        .usingComparatorForType(ResponseBodyMatchers::compareTemporalByDelta, LocalDateTime.class)
+                        .usingRecursiveComparison()
+                        .ignoringFields(ignoredFields.toArray(new String[0]))
+                        .isEqualTo(expectedObject);
+
+                for (var requiredFieldName : fieldNamesToIgnore) {
+                    assertThatCode(
+                            () -> JsonPath.read(json, jsonPath + "." + requiredFieldName)
+                    ).doesNotThrowAnyException();
+                }
+            } catch (Exception e) {
+                fail("Unable to extract result set. Maybe the path does not contain an object of that type?", e);
+            }
+        };
+    }
+
+    // Not every machine has 9 digits precision, so we need to compare with a delta
+    private static int compareTemporalByDelta(OffsetDateTime a, OffsetDateTime b) {
+        return Math.abs(ChronoUnit.NANOS.between(a, b)) < 1000 ? 0 : a.compareTo(b);
+    }
+
+    private static int compareTemporalByDelta(LocalDateTime a, LocalDateTime b) {
+        return Math.abs(ChronoUnit.NANOS.between(a, b)) < 1000 ? 0 : a.compareTo(b);
+    }
+}