在Groovy中，println不使用重写的toString方法

Question

I have overriden toString() for my object:

class GroovyTest {
    static class TestObject {
        int a = 3
    }
    static main(def s) {
        TestObject.metaClass.toString = { -> 'silly'}   
        println new TestObject()
    }
}

Ignoring my custom toString() it uses the default toString() and prints:

GroovyTest$TestObject@195ed659

However if I manually invoke it like println new TestObject().toString() it works:

Silly

Why is this?

Answer 1

When you override a method in Groovy you are not actually modifying the Java byte code for the class. Your method gets added to a metaClass registry. The method calls on the object are intercepted and the metaClass is checked.

When a Groovy object is passed to Java code it will lose all of its Grooviness and become a POJO because method interception and metaClass checking will not happen there. See this post .

println internally uses System.out.println which bypasses the Groovy method interception and invokes Object.toString() .