英文:
golang: json.Unmarshal() returns "invalid memory address or nil pointer dereference"
问题
我从一个 WebSocket 收到了一个 JSON 消息，JSON 字符串接收正常。然后我调用了 json.Unmarshal 函数，但出现了运行时 panic。我查看了其他示例，但这似乎是另外的问题。以下是代码：
func translateMessages(s socket) {
message := make([]byte, 4096)
for {
fmt.Printf("Waiting for a message ... \n")
if n, err := s.Read(message); err == nil {
command := map[string]interface{}{}
fmt.Printf("Received message: %v (%d Bytes)\n", string(message[:n]), n)
err := json.Unmarshal(message[:n], &command)
fmt.Printf("Received command: %v (Error: %s)\n", command, err.Error())
}
}
}
以下是输出结果：
Waiting for a message ...
Received message: {"gruss":"Hello World!"} (24 Bytes)
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x20 pc=0x401938]
goroutine 25 [running]:
runtime.panic(0x6f4860, 0x8ec333)
有什么提示可以解决这个问题吗？
英文:
I get a json message from a websocket an the json string is received ok. Then I call json.Unmarshal an get a runtime panic. I looked through the other examples, but this seems to be something else. Here is the code:
func translateMessages(s socket) {
message := make([]byte,4096)
for {
fmt.Printf("Waiting for a message ... \n")
if n, err := s.Read(message); err == nil {
command := map[string]interface{}{}
fmt.Printf("Received message: %v (%d Bytes)\n", string(message[:n]), n)
err := json.Unmarshal(message[:n],&command)
fmt.Printf("Received command: %v (Error: %s)\n", command, err.Error())
}
}
}
And this is the output:
Waiting for a message ...
Received message: {"gruss":"Hello World!"} (24 Bytes)
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x20 pc=0x401938]
goroutine 25 [running]:
runtime.panic(0x6f4860, 0x8ec333)
Any hint what that could be?
答案1
得分: 2
如果在解码JSON时没有错误，这行代码将会引发panic：
fmt.Printf("Received command: %v (Error: %s)\n", command, err.Error())
如果err == nil，那么err.Error()将会引发空指针解引用的panic。将该行代码改为：
fmt.Printf("Received command: %v (Error: %v)\n", command, err)
如果你正在读取一个socket，那么不能保证s.Read()会读取到一个完整的JSON值。编写这个函数的更好方式是：
func translateMessages(s socket) {
d := json.NewDecoder(s)
for {
fmt.Printf("Waiting for a message ... \n")
var command map[string]interface{}
err := d.Decode(&command)
fmt.Printf("Received command: %v (Error: %v)\n", command, err)
if err != nil {
return
}
}
}
如果你正在使用websockets，那么你应该使用gorilla/websocket包和ReadJSON来解码JSON值。
英文:
This line will panic if there's no error decoding the JSON:
fmt.Printf("Received command: %v (Error: %s)\n", command, err.Error())
If err == nil, then err.Error() panics with nil pointer derference. Change the line to:
fmt.Printf("Received command: %v (Error: %v)\n", command, err)
If you are reading a socket, then there's no guarantee that s.Read() will read a complete JSON value. A better way to write this function is:
func translateMessages(s socket) {
d := json.NewDecoder(s)
for {
fmt.Printf("Waiting for a message ... \n")
var command map[string]interface{}
err := d.Decode(&command)
fmt.Printf("Received command: %v (Error: %v)\n", command, err)
if err != nil {
return
}
}
}
If you are working with websockets, then you should use the gorilla/webscoket package and ReadJSON to decode JSON values.
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