2. ConstantTransformer

ConstantTransformer类是Transformer接口的实现类，其中ConstantTransformer类重写了接口类的transformer方法

public class ClosureTransformer implements Transformer, Serializable {
    private static final long serialVersionUID = 478466901448617286L;
    private final Closure iClosure;
    public static Transformer getInstance(Closure closure) {
        if (closure == null) {
            throw new IllegalArgumentException("Closure must not be null");
        } else {
            return new ClosureTransformer(closure);
        }
    }
    public ClosureTransformer(Closure closure) {
        this.iClosure = closure;
    }
    public Object transform(Object input) {
        this.iClosure.execute(input);
        return input;
    }
    public Closure getClosure() {
        return this.iClosure;
    }
}

通过ConstantTransformer类的transform方法获取一个对象类型，如transform参数是Runtime.class时，调用ConstantTransformer类的transform方法，执行后返回java.lang.Runtime类