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diff --git a/web/blog/posts/make-object-t-again.md b/web/blog/posts/make-object-t-again.md new file mode 100644 index 000000000000..420b57c0fde9 --- /dev/null +++ b/web/blog/posts/make-object-t-again.md @@ -0,0 +1,98 @@ +A few minutes ago I found myself debugging a strange Java issue related +to Jackson, one of the most common Java JSON serialization libraries. + +The gist of the issue was that a short wrapper using some types from +[Javaslang](http://www.javaslang.io/) was causing unexpected problems: + +```java +public <T> Try<T> readValue(String json, TypeReference type) { + return Try.of(() -> objectMapper.readValue(json, type)); +} +``` + +The signature of this function was based on the original Jackson +`readValue` type signature: + +```java +public <T> T readValue(String content, TypeReference valueTypeRef) +``` + +While happily using my wrapper function I suddenly got an unexpected +error telling me that `Object` is incompatible with the type I was +asking Jackson to de-serialize, which got me to re-evaluate the above +type signature again. + +Lets look for a second at some code that will *happily compile* if you +are using Jackson\'s own `readValue`: + +```java +// This shouldn't compile! +Long l = objectMapper.readValue("\"foo\"", new TypeReference<String>(){}); +``` + +As you can see there we ask Jackson to decode the JSON into a `String` +as enclosed in the `TypeReference`, but assign the result to a `Long`. +And it compiles. And it failes at runtime with +`java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long`. +Huh? + +Looking at the Jackson `readValue` implementation it becomes clear +what\'s going on here: + +```java +@SuppressWarnings({ "unchecked", "rawtypes" }) +public <T> T readValue(String content, TypeReference valueTypeRef) + throws IOException, JsonParseException, JsonMappingException +{ + return (T) _readMapAndClose(/* whatever */); +} +``` + +The function is parameterised over the type `T`, however the only place +where `T` occurs in the signature is in the parameter declaration and +the function return type. Java will happily let you use generic +functions and types without specifying type parameters: + +```java +// Compiles fine! +final List myList = List.of(1,2,3); + +// Type is now myList : List<Object> +``` + +Meaning that those parameters default to `Object`. Now in the code above +Jackson also explicitly casts the return value of its inner function +call to `T`. + +What ends up happening is that Java infers the expected return type from +the context of the `readValue` and then happily uses the unchecked cast +to fit that return type. If the type hints of the context aren\'t strong +enough we simply get `Object` back. + +So what\'s the fix for this? It\'s quite simple: + +```java +public <T> T readValue(String content, TypeReference<T> valueTypeRef) +``` + +By also making the parameter appear in the `TypeReference` we \"bind\" +`T` to the type enclosed in the type reference. The cast can then also +safely be removed. + +The cherries on top of this are: + +1. `@SuppressWarnings({ "rawtypes" })` explicitly disables a + warning that would\'ve caught this + +2. the `readValue` implementation using the less powerful `Class` + class to carry the type parameter does this correctly: `public <T> + T readValue(String content, Class<T> valueType)` + +The big question I have about this is *why* does Jackson do it this way? +Obviously the warning did not just appear there by chance, so somebody +must have thought about this? + +If anyone knows what the reason is, I\'d be happy to hear from you. + +PS: Shoutout to David & Lucia for helping me not lose my sanity over +this. |