Synthetic example, but possible real case:

Domain class
---------------------------------------------

public class E3 extends ReflectionDBObject {
private String stringData;
private E4 e4data;

public E3() {
}

public E3(String stringData, E4 e4data)

{ this.stringData = stringData; this.e4data = e4data; }

public String getStringData()

{ return stringData; }

public void setStringData(String stringData)

{ this.stringData = stringData; }

public E4 getE4data()

{ return e4data; }

public void setE4data(E4 e4data)

{ this.e4data = e4data; }

}

Domain class
---------------------------------------------

public class E4 extends ReflectionDBObject {
private int integerData;
private E3 e3data;

public E4() {
}

public E4(int integerData, E3 e3data)

{ this.integerData = integerData; this.e3data = e3data; }

public int getIntegerData()

{ return integerData; }

public void setIntegerData(int integerData)

{ this.integerData = integerData; }

public E3 getE3data()

{ return e3data; }

public void setE3data(E3 e3data)

{ this.e3data = e3data; }

}

Application code
---------------------------------------------

E3 e3 = new E3();
E4 e4 = new E4(4, e3);
e3.setE4data(e4);
e3.setStringData("e3");

coll.save(e3);

---------------------------------------------

Exception in thread "main" java.lang.StackOverflowError
...
at com.mongodb.ReflectionDBObject$JavaWrapper.get(ReflectionDBObject.java:155)
at com.mongodb.ReflectionDBObject.get(ReflectionDBObject.java:31)
at com.mongodb.DBCollection._checkKeys(DBCollection.java:852)
at com.mongodb.DBCollection._checkKeys(DBCollection.java:853)
at com.mongodb.DBCollection._checkKeys(DBCollection.java:853)
...