handle typed nil pointers in Encode method (apache/dubbo-go#2517)

[Nexusrex18]

This PR fixes the issue where the Encode method in encode.go did not properly handle typed nil pointers (e.g., (*int32)(nil)). The changes ensure that such pointers are encoded as Hessian NULL ('N').

Changes:

• Modified encode.go to add a reflect-based check for typed nil pointers.
• Added tests in encode_test.go to verify the behavior for typed nil pointers, untyped nil, and non-nil pointers.
• Fixed TestUserDefinedException in decode_test.go by:
• Updating getJavaReply to handle non-zero exit statuses from the Java program.
• Correcting the Java class name to com.test.UserDefinedException and updating TestThrowable.java accordingly.

Fixes apache/dubbo-go#2517 (Ref: apache/dubbo-go#2517)

This PR is a prerequisite for the corresponding changes in apache/dubbo-go.

[tiltwind]

why ignore the the exec.ExitError?

[tiltwind]

we define *int32 as java.lang.Integer,

dubbo-go-hessian2/java_lang_test.go

Line 60 in e93635a

got, err := decodeJavaResponse(`customReplyJavaIntegerArray`, ``, false)

[mochengqian]

@mochengqian

[AlexStocks]

Code Review: handle typed nil pointers in Encode method (apache/dubbo-go#2517)

评价

问题定位准确，修复方案正确。解决了 (*int32)(nil) 这类 typed nil pointer 无法正确编码为 Hessian null 的问题。

[P1] 规范级问题

1. encode.go:102-109 - 性能开销

使用 reflect.ValueOf(v).Kind() 和 IsNil() 会增加性能开销。对于高频调用的 Encode 方法，建议使用类型断言优化：

// 优化方案：优先使用类型断言
switch v.(type) {
case nil:
    e.buffer = EncNull(e.buffer)
    return nil
default:
    vVal := reflect.ValueOf(v)
    if vVal.Kind() == reflect.Ptr && vVal.IsNil() {
        e.buffer = EncNull(e.buffer)
        return nil
    }
}
// 继续原有类型判断...

2. 与 PR #384 的关系

本 PR 与 #384 解决同一问题且修改相同文件，建议：

• 合并两个 PR
• 保留更完整的测试覆盖

3. 测试覆盖

encode_test.go 的测试覆盖了主要类型，但建议补充：

• 嵌套指针类型（**int32）
• 接口类型的 nil 判断
• 结构体指针的 nil 判断

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总体评价：修复正确有效，建议合并 #376 和 #384，并补充嵌套指针类型测试。