Hidden exceptions
- by user12617285
Occasionally you may find yourself in a Java application environment where exceptions in your code are being caught by the application framework and either silently swallowed or converted into a generic exception. Either way, the potentially useful details of your original exception are inaccessible. Wouldn't it be nice if there was a VM option that showed the stack trace for every exception thrown, whether or not it's caught? In fact, HotSpot includes such an option: -XX:+TraceExceptions. However, this option is only available in a debug build of HotSpot (search globals.hpp for TraceExceptions). And based on a quick skim of the HotSpot source code, this option only prints the exception class and message. A more useful capability would be to have the complete stack trace printed as well as the code location catching the exception. This is what the various TraceException* options in in Maxine do (and more). That said, there is a way to achieve a limited version of the same thing with a stock standard JVM. It involves the use of the -Xbootclasspath/p non-standard option. The trick is to modify the source of java.lang.Exception by inserting the following:
private static final boolean logging = System.getProperty("TraceExceptions") != null;
private void log() {
if (logging && sun.misc.VM.isBooted()) {
printStackTrace();
}
}
Then every constructor simply needs to be modified to call log() just before returning:
public Exception(String message) {
super(message);
log();
}
public Exception(String message, Throwable cause) {
super(message, cause);
log();
}
// etc...
You now need to compile the modified Exception.java source and prepend the resulting class to the boot class path as well as add -DTraceExceptions to your java command line. Here's a console session showing these steps:
% mkdir boot
% javac -d boot Exception.java
% java -DTraceExceptions -Xbootclasspath/p:boot -cp com.oracle.max.vm/bin test.output.HelloWorld
java.util.zip.ZipException: error in opening zip file
at java.util.zip.ZipFile.open(Native Method)
at java.util.zip.ZipFile.(ZipFile.java:127)
at java.util.jar.JarFile.(JarFile.java:135)
at java.util.jar.JarFile.(JarFile.java:72)
at sun.misc.URLClassPath$JarLoader.getJarFile(URLClassPath.java:646)
at sun.misc.URLClassPath$JarLoader.access$600(URLClassPath.java:540)
at sun.misc.URLClassPath$JarLoader$1.run(URLClassPath.java:607)
at java.security.AccessController.doPrivileged(Native Method)
at sun.misc.URLClassPath$JarLoader.ensureOpen(URLClassPath.java:599)
at sun.misc.URLClassPath$JarLoader.(URLClassPath.java:583)
at sun.misc.URLClassPath$3.run(URLClassPath.java:333)
at java.security.AccessController.doPrivileged(Native Method)
at sun.misc.URLClassPath.getLoader(URLClassPath.java:322)
at sun.misc.URLClassPath.getLoader(URLClassPath.java:299)
at sun.misc.URLClassPath.getResource(URLClassPath.java:168)
at java.net.URLClassLoader$1.run(URLClassLoader.java:194)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at sun.misc.Launcher$ExtClassLoader.findClass(Launcher.java:229)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:295)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
java.security.PrivilegedActionException
at java.security.AccessController.doPrivileged(Native Method)
at sun.misc.URLClassPath$JarLoader.ensureOpen(URLClassPath.java:599)
at sun.misc.URLClassPath$JarLoader.(URLClassPath.java:583)
at sun.misc.URLClassPath$3.run(URLClassPath.java:333)
at java.security.AccessController.doPrivileged(Native Method)
at sun.misc.URLClassPath.getLoader(URLClassPath.java:322)
...
It's worth pointing out that this is not as useful as direct VM support for tracing exceptions. It has (at least) the following limitations:
The trace is shown for every exception, whether it is thrown or not.
It only applies to subclasses of java.lang.Exception as there appears to be bootstrap issues when the modification is applied to Throwable.java.
It does not show you where the exception was caught.
It involves overriding a class in rt.jar, something should never be done in a non-development environment.
