Java7引导程序：检查类而不加载？ [英] Java7 bootstrap: Checking class without loading?

问题描述

在阅读这个问题的答案时，我想知道Java7引导程序是如何知道 public static void main（String [] args）方法，而不运行静态初始化程序的？我对这个主题有一些假设，但其中一些显然是错误的：

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:

• Java Bootstrap在JVM中运行，所以它只能使用标准的JVM特性（没有本机特性） - 被调用的类必须在CLASSPATH上，例如


• 标准的JVM类加载是通过普通的类加载机制完成的（我知道它有几个步骤，我已经多次使用类加载器了。）


• 在解析（链接）类之后，立即运行类初始化（包括初始化静态属性和运行静态初始化程序）。


• 没有办法分开前两个步骤

• Java Bootstrap is running in JVM, so it can only use standard JVM features (no native features) - the called class must be on CLASSPATH, for example
• The standard JVM classloading is done via the normal classloading mechanism (I know it has several steps, I've been playing with classloaders several times)
• After the class has been resolved (linked), the Class initialization is run immediately (including initializing static attributes and running static initializers)
• There is no way to part the previous two steps

我的问题是：

• 谁调用类初始值设定项以及在什么阶段？ （之前会发生什么，之​​后会发生什么？）


• 为什么引导程序与普通类加载相比表现不同？对此有更多的区别吗？


• 奖金问题：我的哪些假设是错误的？

总结提到的问题：如果您运行Java主类（来自Java7命令行），它将检查是否存在 main（）方法，而不运行静态初始化。在Java6中，它的行为有所不同。

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.

推荐答案

可以在不链接的情况下加载类：Java 8中的启动程序加载主class调用类加载器上的loadClass 方法，它不链接类或运行任何静态方法。

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.

参见 http://hg.openjdk.java.net /jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/sun/launcher/LauncherHelper.java#l495 和
java.c＃l1221，用于在Java中加载主类的代码8。

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.

Java 6启动程序使用FindClass JNI方法加载主类：我猜这可以解释行为上的差异。请参阅jdk6 / jdk6 / jdk / file / 814bf0775b52 / src / share / bin / java.c＃l1387

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确实初始化了类，至少在Java 6上是这样的：在jdk6 / jdk6 / hotspot / file / 91e5cc5d33b9 / src / share / vm / prims / jni.cpp中的实现

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）;

调用，在jdk6 / jdk6 / hotspot / file / 91e5cc5d33b9 / src / share / vm / prims / jvm_misc.hpp，

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）;

，init为true

with init as true

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