✅ Add JSONBodyHandler and tests

- Implemented JSONBodyHandler for automatic JSON body parsing.
- Added comprehensive unit tests for JSONBodyHandler covering successful decoding, invalid JSON, empty bodies, and error propagation.
- Refactored handler_test.go to improve test structure and resolve linting issues (funlen, gocritic).

handler.go

@@ -1,6 +1,7 @@
 package ezhandler
 
 import (
+	"encoding/json"
 	"io"
 	"net/http"
 )
@@ -42,3 +43,22 @@ func (fn ResponseHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) erro
 	_, err = io.Copy(w, body)
 	return err
 }
+
+// JSONBodyHandler can be used as a [Handler] which automatically parses the json body into a value, which is passed to the function.
+type JSONBodyHandler[V any] func(w http.ResponseWriter, r *http.Request, body V) error
+
+var _ Handler = JSONBodyHandler[map[string]any](nil)
+
+func (fn JSONBodyHandler[V]) ServeHTTP(w http.ResponseWriter, r *http.Request) error {
+	reqBody := r.Body
+	//nolint:errcheck // This is usually fine
+	defer reqBody.Close()
+
+	dec := json.NewDecoder(reqBody)
+	var body V
+	if err := dec.Decode(&body); err != nil {
+		return err
+	}
+
+	return fn(w, r, body)
+}

handler_test.go

@@ -1,8 +1,10 @@
 package ezhandler_test
 
 import (
+	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"codeberg.org/danjones000/ezhandler"
@@ -46,3 +48,99 @@ func TestHandlerFunc_ServeHTTP(t *testing.T) {
 		})
 	}
 }
+
+type testBody struct {
+	Name string `json:"name"`
+	Age  int    `json:"age"`
+}
+
+type jsonBodyTest struct {
+	name                string
+	requestBody         string
+	handler             ezhandler.JSONBodyHandler[testBody]
+	expectedStatus      int
+	expectedBody        string
+	expectedErr         error
+	expectedErrContains string
+	additionAssert      func(*assert.Assertions, testBody)
+}
+
+var jsonBodyHandlerTests = []jsonBodyTest{
+	{
+		name:        "successful decoding and handler execution",
+		requestBody: `{"name":"John Doe","age":30}`,
+		handler: ezhandler.JSONBodyHandler[testBody](func(w http.ResponseWriter, r *http.Request, body testBody) error {
+			w.WriteHeader(http.StatusOK)
+			_, _ = w.Write([]byte("success"))
+			return nil
+		}),
+		expectedStatus: http.StatusOK,
+		expectedBody:   "success",
+		expectedErr:    nil,
+		additionAssert: func(as *assert.Assertions, body testBody) {
+			as.Equal("John Doe", body.Name)
+			as.Equal(30, body.Age)
+		},
+	},
+	{
+		name:        "invalid JSON body",
+		requestBody: `{"name":"John Doe","age":}`,
+		handler: ezhandler.JSONBodyHandler[testBody](func(w http.ResponseWriter, r *http.Request, body testBody) error {
+			return nil // Should not be called
+		}),
+		expectedErrContains: "invalid character",
+	},
+	{
+		name:        "empty body for struct type",
+		requestBody: "",
+		handler: ezhandler.JSONBodyHandler[testBody](func(w http.ResponseWriter, r *http.Request, body testBody) error {
+			return nil // Should not be called
+		}),
+		expectedErr: io.EOF,
+	},
+	{
+		name:        "handler function returns error",
+		requestBody: `{"name":"Jane Doe","age":25}`,
+		handler: ezhandler.JSONBodyHandler[testBody](func(w http.ResponseWriter, r *http.Request, body testBody) error {
+			return errTest
+		}),
+		expectedErr: errTest,
+	},
+}
+
+func TestJSONBodyHandler_ServeHTTP(t *testing.T) {
+	for _, tc := range jsonBodyHandlerTests {
+		runJSONBodyHandlerTest(t, tc)
+	}
+}
+
+func runJSONBodyHandlerTest(tt *testing.T, tc jsonBodyTest) {
+	tt.Run(tc.name, func(t *testing.T) {
+		handler := ezhandler.JSONBodyHandler[testBody](func(w http.ResponseWriter, r *http.Request, body testBody) error {
+			if err := tc.handler(w, r, body); err != nil {
+				return err
+			}
+			if tc.additionAssert != nil {
+				as := assert.New(t)
+				tc.additionAssert(as, body)
+			}
+			return nil
+		})
+		req := httptest.NewRequest(http.MethodPost, "/", strings.NewReader(tc.requestBody))
+		rec := httptest.NewRecorder()
+
+		err := handler.ServeHTTP(rec, req)
+
+		switch {
+		case tc.expectedErr != nil:
+			assert.Equal(t, tc.expectedErr, err)
+		case tc.expectedErrContains != "":
+			assert.ErrorContains(t, err, tc.expectedErrContains)
+		default:
+			assert.NoError(t, err)
+			assert.Equal(t, tc.expectedStatus, rec.Code)
+			assert.Equal(t, tc.expectedBody, rec.Body.String())
+		}
+
+	})
+}