英文:
Why is NPE not reported on correct line
问题
运行以下代码：
public class NPESample {
String value;
public static void main(String[] args) {
NPESample npeSample = null;
"foo"
.replaceAll(
"f",
npeSample.value);
}
}
将会得到：
Exception in thread "main" java.lang.NullPointerException
at NPESample.main(NPESample.java:9)
报告的行数 (7) 是 "foo" 这一行，而不是实际发生 NPE 的行数 (10) npeSample.value。
为什么会这样？当我调试代码时，被报告的行数与代码中实际发生问题的行数不一致，这让我感到困惑。
要复现这个问题，运行以下代码：
cat > ./NPESample.java <<DELIM
public class NPESample {
String value;
public static void main(String[] args) {
NPESample npeSample = null;
"foo"
.replaceAll(
"f",
npeSample.value);
}
}
DELIM
javac NPESample.java
java NPESample
输出结果为：
Exception in thread "main" java.lang.NullPointerException
at NPESample.main(NPESample.java:7)
javac -version
javac 13.0.1
java -version
openjdk version "13.0.1" 2019-10-15
OpenJDK Runtime Environment (build 13.0.1+9)
OpenJDK 64-Bit Server VM (build 13.0.1+9, mixed mode, sharing)
英文:
Running the following:
public class NPESample {
String value;
public static void main(String[] args) {
NPESample npeSample = null;
"foo"
.replaceAll(
"f",
npeSample.value);
}
}
Will give:
Exception in thread "main" java.lang.NullPointerException
at NPESample.main(NPESample.java:9)
Reported line (7) is the line "foo" and not the actual line where NPE happens (10) npeSample.value.
Why is this? I was caught by surprise when debugging code and thrown off by the "wrong" reported line number in the code.
To reproduce run this:
cat > ./NPESample.java <<DELIM
public class NPESample {
String value;
public static void main(String[] args) {
NPESample npeSample = null;
"foo"
.replaceAll(
"f",
npeSample.value);
}
}
DELIM
javac NPESample.java
java NPESample
output is:
Exception in thread "main" java.lang.NullPointerException
at NPESample.main(NPESample.java:7)
javac -version
javac 13.0.1
java -version
openjdk version "13.0.1" 2019-10-15
OpenJDK Runtime Environment (build 13.0.1+9)
OpenJDK 64-Bit Server VM (build 13.0.1+9, mixed mode, sharing)
答案1
得分: 2
这在我看来并不像是一个 bug。具体来说，它似乎允许映射不必是一对一的，正如规范所述：
也就是说，LineNumberTable 属性可以共同表示源文件的某一行，并且不一定与源代码行一一对应。
甚至 JDK-15 报告了错误的行：
无法读取字段“value”，因为“npeSample”为空
位于第 9 行。
英文:
This does not look like a bug, to me. Specifically, it seems that the mapping is freely allowed not be one-one, as the SPEC says:
> That is, LineNumberTable attributes may together represent a given line of a source file, and need not be one-to-one with source lines.
Even JDK-15 reports the wrong line:
> Cannot read field "value" because "npeSample" is null
on line 9.
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