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Java谷園から逃げ出した
話
Kotlin愛好会 vol.24
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About Me
● Matthew Vern / Panini
● Twitter: @panini_ja Github: panpanini
● Mercari, Inc
● Android Engineer (US)
● Kotlin可愛い
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Summary
● Protobuf is a type-safe alternative to json
● Most protobuf implementations only generate Java
● NullPointerException
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Protocol Buffers
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Protocol Buffers
● https://developers.google.com/protocol-buffers
● Google's language-neutral, platform-neutral, extensible mechanism for
serializing structured data
● Defines API contract between Server and Client
● Proto models are generated per platform needed
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Protocol Buffers - messages
● Message is a definition of an object you want to share
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Protocol Buffers - messages
● Proto3 syntax:
○ https://developers.google.com/protocol-buffers/docs/proto3
○ When no value is set, return a Default Value
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Protogen Java
● (in Jan 2019) Only 2 ways to generate proto for Android
○ https://github.com/square/wire
○ https://developers.google.com/protocol-buffers/docs/reference/java-generated
● Neither use Nullability annotations
⇒ NullPointerException
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Protogen Java - message
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Protogen Java - message
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Problems
● Generated code doesn’t provide Nullability annotations
○ Because no annotations, Kotlin will use platform types (eg: String!)
● Both Wire & Google protogen-java return null by default, and require callers to
check to replace Default Value
○ Eg: Wire.get(person.name, Person.DEFAULT_NAME)
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Protok
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Protok
● protoc plugin to generate Kotlin
● Null safety
● Message as data class
● https://github.com/panpanini/protok
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Protok - message
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Summary
● With Kotlin, we could avoid NPE Java谷園
● No more crashes with Protok!
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Protokがクラッシュした話
全部Java(JVM)が悪い(???)
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Summary
● Mercari US has used Protok in production for 1.5 years with no problems
● Suddenly, there were problems
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The problem
● After updating our proto definitions, the app would suddenly crash
● java.lang.VerifyError: Verifier rejected class
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Error vs Exception
● Error: "indicates serious problems that an application should not try to catch."
● Exception: "indicates conditions that an application should catch."
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VerifyError
● https://docs.oracle.com/javase/7/docs/api/java/lang/VerifyError.html
● Thrown when the "verifier" detects that a class file, though well formed, contains
some sort of internal inconsistency or security problem.
● Not a compile error, so probably we found a limitation of the JVM
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JVM limitations
● https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.11
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JVM limitations
● https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.11
● The number of method parameters is limited to 255 by the definition of a method
descriptor (§4.3.3), where the limit includes one unit for this in the case of
instance or interface method invocations.
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Problems
● Protok uses the constructor to pass fields to the message instance
● Constructor is a function
● A function can only have 255* parameters
⇒ A protok message can only have 255 fields
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Solution
● Protok no longer uses data class
● https://github.com/panpanini/protok/pull/64
● Use Builder pattern & function with lambda to make a better Kotlin API
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Solution
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Summary
● Protok provides null safety for proto3 messages
● JVM only supports 255 method parameters
● Error and Exception are different in Java
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Have a nice Kotlin!