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