How would I print the JVM's system properties

问题:

You can obtain a Properties instance of the JVM properties using System.getProperties(); how would you go about using Java 8 code to print all properties to the console?

回答1:

One solution:

public final class Foo
{
    private static void printProperty(final Object key, final Object value)
    {
        System.out.println(key + ": " + value);
    }

    public static void main(final String... args)
    {
        System.getProperties().forEach(Foo::printProperty);
    }
}

Rundown:

• Properties extends Hashtable<Object, Object> which itself implements Map<Object, Object>;
• Map has a .forEach() method whose argument is a BiConsumer;
• BiConsumer is a functional interface;
• static method printProperty() of class Foo happens to have the same signature as a BiConsumer<Object, Object>: its "return value" is void, its first argument is Object, its second argument is Object;
• we can therefore use Foo::printProperty as a method reference.

A shorter version would be:

public final class ShorterFoo
{
    public static void main(final String... args)
    {
        System.getProperties()
            .forEach((key, value) -> System.out.println(key + ": " + value));
    }
}

At runtime, this would not make a difference. Note the type inference in the second example: the compiler can infer that key and value are of type Object. Another way to write this "anonymous lambda" would have been:

(Object key, Object value) -> System.out.println(key + ": " + value)

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(not so) Side note: even though it is a little outdated, you really want to watch this video (yes, it's one hour long; yes, it is worth watching it all).

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(not so) Side note 2: you may have noticed that Map's .forEach() mentions a default implementation; this means that your custom Map implementations, or other implementations from external libraries, will be able to use .forEach() (for instance, Guava's ImmutableMaps). Many such methods on Java collections exist; do not hesitate to use these "new methods" on "old dogs".

回答2:

@fge has missed one very short version that admittedly depends on the toString implementation of Map.Entry.

public class VeryShortFoo {
    public static void main(String... args) {
        System.getProperties().entrySet().forEach(System.out::println);
    }
}

• Here, the entrySet is streamed and each Map.Entry is printed with a reference to out.println.
• Map.Entry implementations of toString generally return getKey() + "=" + getValue().

Here's another one I quite like.

public class ElegantFoo {
    public static void main(String... args) {
        System.getProperties().entrySet().stream()
            .map(e -> e.getKey() + ": " + e.getValue())
            .forEach(System.out::println);
    }
}

• The entrySet is streamed again (this time explicitly with a call to stream).
• Stream#map performs a 1:1 conversion from elements of one type to elements of another. Here, it turns a Stream<Map.Entry> in to a Stream<String>.
• The Stream<String> is printed.

回答3:

In Java 8, the Properties class inherits a new method from HashTable called forEach. This new method accepts functions (functional interfaces) to be passed to it as arguments. To be more specific, it accepts the functional interface BiConsumer<T,U>. This functional interface's functional method is accept(T t, U u). In Java 8, all functional interfaces can be written as Lambda expressions. Therefore, here is how we would display all properties in a Property instance:

Properties vmProps = System.getProperties();
vmProps.forEach((t,u) -> System.out.println("Property: " + t + "\nValue: " + u + "\n"));

回答4:

Sorted by key (makes it useful)

printProperties(System.getProperties());

public static void printProperties(Properties properties) {
    new TreeSet<>(properties.keySet()).forEach((k) -> {
        System.out.println(k + " : " + properties.get(k));
    });
}

回答5:

System.getProperties().list(System.out);