feat(cache): 值编解码器(fastjson 序列化 + 空值标记)

src/main/java/com/njcn/middle/cache/support/CacheValueCodec.java

@@ -0,0 +1,37 @@
+package com.njcn.middle.cache.support;
+
+import com.alibaba.fastjson.JSON;
+import com.alibaba.fastjson.TypeReference;
+import com.njcn.middle.cache.constant.CacheConstant;
+
+/**
+ * 缓存值编解码:对象 ↔ 字符串(fastjson),并承载空值标记。
+ *
+ * <p>解码失败直接抛 fastjson 异常({@link RuntimeException}),由调用方决定显式暴露(public get)
+ * 还是当未命中降级(getOrLoad 内部)。
+ */
+public class CacheValueCodec {
+
+    /** 对象 → 字符串:null → 空值标记;否则 fastjson 序列化。 */
+    public String encode(Object value) {
+        if (value == null) {
+            return CacheConstant.NULL_SENTINEL;
+        }
+        return JSON.toJSONString(value);
+    }
+
+    /** 是否空值标记。 */
+    public boolean isNullSentinel(String raw) {
+        return CacheConstant.NULL_SENTINEL.equals(raw);
+    }
+
+    /** 字符串 → 对象(Class)。 */
+    public <T> T decode(String raw, Class<T> type) {
+        return JSON.parseObject(raw, type);
+    }
+
+    /** 字符串 → 对象(TypeReference,支持泛型)。 */
+    public <T> T decode(String raw, TypeReference<T> type) {
+        return JSON.parseObject(raw, type);
+    }
+}

src/test/java/com/njcn/middle/cache/support/CacheValueCodecTest.java

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+package com.njcn.middle.cache.support;
+
+import com.alibaba.fastjson.TypeReference;
+import com.njcn.middle.cache.constant.CacheConstant;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class CacheValueCodecTest {
+
+    private final CacheValueCodec codec = new CacheValueCodec();
+
+    @Data
+    @AllArgsConstructor
+    @NoArgsConstructor
+    static class Foo {
+        private Long id;
+        private String name;
+    }
+
+    @Test
+    void encodeNullReturnsSentinel() {
+        assertEquals(CacheConstant.NULL_SENTINEL, codec.encode(null));
+        assertTrue(codec.isNullSentinel(codec.encode(null)));
+        assertFalse(codec.isNullSentinel("{\"id\":1}"));
+    }
+
+    @Test
+    void encodeDecodePojoRoundTrip() {
+        Foo foo = new Foo(1L, "alice");
+        String raw = codec.encode(foo);
+        Foo back = codec.decode(raw, Foo.class);
+        assertEquals(foo, back);
+    }
+
+    @Test
+    void encodeDecodeStringRoundTrip() {
+        String raw = codec.encode("abc");
+        assertEquals("abc", codec.decode(raw, String.class));
+    }
+
+    @Test
+    void decodeGenericWithTypeReference() {
+        List<Integer> list = Arrays.asList(1, 2, 3);
+        String raw = codec.encode(list);
+        List<Integer> back = codec.decode(raw, new TypeReference<List<Integer>>() {
+        });
+        assertEquals(list, back);
+    }
+
+    @Test
+    void decodeDirtyStringThrows() {
+        assertThrows(RuntimeException.class, () -> codec.decode("{not-json", Foo.class));
+    }
+}