jvm实用参数

开启类加载日志

-verbose:class，如java -verbose:class xxx，就会开启了jvm加载类的日志输出，例如观察依赖类加载是线程安全的特性来实现延迟加载的单例：

public class Singleton {
  private Singleton() {}
  private static class LazyHolder {
    static final Singleton INSTANCE = new Singleton();
    static {
      System.out.println("LazyHolder.<clinit>");
    }
  }
  public static Object getInstance(boolean flag) {
    if (flag) return new LazyHolder[2];
    return LazyHolder.INSTANCE;
  }
  public static void main(String[] args) {
    getInstance(true);
    System.out.println("----");
    getInstance(false);
  }
}
运行： java -verbose:class Singleton，输出如下log：
[Opened /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.Object from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.io.Serializable from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.Comparable from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.CharSequence from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.String from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.reflect.AnnotatedElement from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.reflect.GenericDeclaration from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.reflect.Type from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.Class from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
.
.
.
[Loaded java.security.BasicPermissionCollection from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded Singleton from file:/home/mcloud/test/]
[Loaded sun.launcher.LauncherHelper$FXHelper from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.Class$MethodArray from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.Void from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded Singleton$LazyHolder from file:/home/mcloud/test/]
----
LazyHolder.<clinit>
[Loaded java.lang.Shutdown from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]
[Loaded java.lang.Shutdown$Lock from /usr/lib/jvm/jdk1.8.0_92/jre/lib/rt.jar]