import sun.tracing.NullProviderFactory; //导入依赖的package包/类
/**
 * Returns an implementation of a {@code ProviderFactory} which
 * creates instances of Providers.
 *
 * The created Provider instances will be linked to all appropriate
 * and enabled system-defined tracing mechanisms in the JDK.
 *
 * @return a {@code ProviderFactory} that is used to create Providers.
 */
public static ProviderFactory getDefaultFactory() {
    HashSet<ProviderFactory> factories = new HashSet<ProviderFactory>();

    // Try to instantiate a DTraceProviderFactory
    String prop = AccessController.doPrivileged(
        new GetPropertyAction("com.sun.tracing.dtrace"));

    if ( (prop == null || !prop.equals("disable")) &&
         DTraceProviderFactory.isSupported() ) {
        factories.add(new DTraceProviderFactory());
    }

    // Try to instantiate an output stream factory
    prop = AccessController.doPrivileged(
        new GetPropertyAction("sun.tracing.stream"));
    if (prop != null) {
        for (String spec : prop.split(",")) {
            PrintStream ps = getPrintStreamFromSpec(spec);
            if (ps != null) {
                factories.add(new PrintStreamProviderFactory(ps));
            }
        }
    }

    // See how many factories we instantiated, and return an appropriate
    // factory that encapsulates that.
    if (factories.size() == 0) {
        return new NullProviderFactory();
    } else if (factories.size() == 1) {
        return factories.toArray(new ProviderFactory[1])[0];
    } else {
        return new MultiplexProviderFactory(factories);
    }
}