java reflection System.out.println

问题

I m learning java reflection. I tried

System.exit(3);
Class.forName("java.lang.System").getMethod("exit", Integer.TYPE).invoke(null, 3);

and it works. I successfully also ran

System.out.println(Class.forName("java.lang.System").getMethod("currentTimeMillis").invoke(null));

Now how can i invoke System.out.println reflectively

java.lang.Class.forName("java.lang.System").getMethod("out.println", String.class).invoke(null, "Hi!");

is giving error. I know System does not have out function. So suggest a way to call System.out.println reflectively

Here is the complete example

public class ReflectionDemo1 {
public static void main(String[] args) throws Exception {
// java.lang.System.exit(3);
// java.lang.Class.forName("java.lang.System").getMethod("exit", Integer.TYPE).invoke(null, 3);
// java.lang.System.currentTimeMillis()
// System.out.println(Class.forName("java.lang.System").getMethod("currentTimeMillis").invoke(null));
// System.out.println("Hi!");
java.lang.Class.forName("java.lang.System").getMethod("out.println", String.class).invoke(null, "Hi!");
}
}

回答1:

out is a static field of the java.lang.System class.

You can use the Field class to refer to it

Class<?> systemClass = java.lang.Class.forName("java.lang.System");
Field outField = systemClass.getDeclaredField("out");

The type of that field is PrintStream. You don't need to know that really, but you do need to retrieve the corresponding Class object for it.

Class<?> printStreamClass = outField.getType();

We know it has a println(String) method, so we can retrieve that too

Method printlnMethod = printStreamClass.getDeclaredMethod("println", String.class);

Now, since println(String) is an instance method, we need to invoke it on an instance. Which instance? The one referenced by the out field. That out field is static so we can get it by passing null to Field#get(object).

Object object = outField.get(null);

Then we invoke the method

printlnMethod.invoke(object, "Hi!");

来源：https://stackoverflow.com/questions/23019482/java-reflection-system-out-println

标签

java

reflection