英文:
What is the meaning of `int dummy` in the fillInStackTrace() method in Java?
问题
private native Throwable fillInStackTrace(int dummy);
这个方法在创建异常时使用dummy=0来调用。dummy的含义是什么？它是要构造的堆栈跟踪的深度吗？
更新：
public class MyEx extends RuntimeException{@Overridepublic synchronized Throwable fillInStackTrace() {Method method = null;try {Class<?>[] classArray = new Class<?>[1];classArray[0] = int.class;method = Throwable.class.getDeclaredMethod("fillInStackTrace", classArray);method.setAccessible(true);Object obj = method.invoke(this, 6);StackTraceElement[] trace = ((MyEx) obj).getStackTrace();System.out.println();} catch (NoSuchMethodException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} catch (InvocationTargetException e) {e.printStackTrace();}return this;}}
看起来dummy真的是虚拟的，无论我放入什么值，结果都是一样的...
我想将堆栈跟踪的大小限制为3，以减少内存消耗，从执行的角度来看，创建异常也会更快。我有一个真实的用例，我需要很多浅层堆栈跟踪的异常。
英文:
private native Throwable fillInStackTrace(int dummy);
This method is called with dummy=0 when an Exception is created. What is the meaning of dummy? Is it the depth of the stack trace to construct?
Update:
public class MyEx extends RuntimeException{@Overridepublic synchronized Throwable fillInStackTrace() {Method method = null;try {Class<?>[] classArray = new Class<?>[1];classArray[0] = int.class;method =Throwable.class.getDeclaredMethod("fillInStackTrace", classArray);method.setAccessible(true);Object obg = method.invoke(this, 6);StackTraceElement[] trace = ((MyEx) obg).getStackTrace();System.out.println();} catch (NoSuchMethodException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} catch (InvocationTargetException e) {e.printStackTrace();}return this;}}
It seems that dummy is really dummy, it doesn't matter what value I put in, the result will be the same...
I wanted to limit the stack trace size to 3 to consume less memory and also creating exceptions would be faster from an execution point of view. I have a real use case where I need many exceptions but with shallow stack traces.
答案1
得分: 1
它没有任何意义。这是一个虚拟参数。
本地代码实现完全忽略了这个参数。例如，在OpenJDK 11中，桥接方法的实现如下：
/** 在这个异常中填入当前堆栈跟踪。通常在创建异常时自动调用，但用户也可以显式调用此方法。此例程返回`this'，因此您可以编写`throw e.fillInStackTrace();'*/JNIEXPORT jobject JNICALLJava_java_lang_Throwable_fillInStackTrace(JNIEnv *env,jobject throwable, jint dummy){JVM_FillInStackTrace(env, throwable);return throwable;}
如您所见，dummy 参数被忽略。
如果您想要限制堆栈跟踪的深度，支持的方法是使用 -XX:MaxJavaStackTraceDepth=depth 选项。
英文:
It doesn't mean anything. It is a dummy argument.
The native code implementation completely ignores the argument. For example, in OpenJDK 11, the bridge method implementation is as follows:
/** Fill in the current stack trace in this exception. This is* usually called automatically when the exception is created but it* may also be called explicitly by the user. This routine returns* `this' so you can write 'throw e.fillInStackTrace();'*/JNIEXPORT jobject JNICALLJava_java_lang_Throwable_fillInStackTrace(JNIEnv *env,jobject throwable, jint dummy){JVM_FillInStackTrace(env, throwable);return throwable;}
As you can see, the dummy argument is ignored.
If you are looking for a way to limit the depth of a stacktrace, the supported way to do this is to use the -XX:MaxJavaStackTraceDepth=depth option.
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