CODEWITHSUNDEEP
javainitializationclassloaderjava-7
gaborsch
gaborsch

Reputation: 15748

Java7 bootstrap: Checking class without loading?

When reading the answer to this question, I was wondering how Java7 bootstrap knows about the presence of public static void main(String[] args) method, without running the static initializers? I have some assumptions on this topic, but some of them is obviously wrong:

My questions are:

To summarize the referred question: if you run a Java main class (from Java7 command line), it will check the presence of the main() method, without running the static initializers. In Java6 it behaves differently.

Upvotes: 2

Views: 432

Answers (1)

user5314618
user5314618

Reputation: 56

It is possible to load a class without linking it: the launcher in Java 8 loads the main class by calling the loadClass method on the classloader, which does not link the class or run any of its static methods.

See http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/sun/launcher/LauncherHelper.java#l495 and java.c#l1221 for the code that loads the main class in Java 8.

The Java 6 launcher uses the FindClass JNI method to load the main class: I'm guessing that this accounts for the difference in behaviour. See jdk6/jdk6/jdk/file/814bf0775b52/src/share/bin/java.c#l1387

FindClass does initialize the class, at least on Java 6: the implementation in jdk6/jdk6/hotspot/file/91e5cc5d33b9/src/share/vm/prims/jni.cpp

result = find_class_from_class_loader(env, sym, true, loader, protection_domain, true, thread);

calls, in jdk6/jdk6/hotspot/file/91e5cc5d33b9/src/share/vm/prims/jvm_misc.hpp,

jclass find_class_from_class_loader(JNIEnv* env, Symbol* name, jboolean init, Handle loader, Handle protection_domain, jboolean throwError, TRAPS);

with init as true

Upvotes: 4

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