互相依赖引用，toString() 导致StackOverflowError

Exception in thread "main" java.lang.StackOverflowError

public class A {
    private B b;
    public B getB() {
        return b;
    }
    public void setB(B b) {
        this.b = b;
    }
    public A(B b) {
        this.b = b;
    }
    public A() {
    }
    @Override
    public String toString() {
        return "A{" +
                "b=" + b +
                '}';
    }
}

public class B {
    private A a;
    public A getA() {
        return a;
    }
    public void setA(A a) {
        this.a = a;
    }
    public B(A a) {
        this.a = a;
    }
    public B() {
    }
    @Override
    public String toString() {
        return "B{" +
                "a=" + a +
                '}';
    }
}

public class StackDemo {

    public static void main(String[] args) {
        A a = new A();
        B b = new B(a);
        a.setB(b);
        a.toString();
    }
}

Exception in thread "main" java.lang.StackOverflowError
at java.lang.StringBuilder.append(StringBuilder.java:136)
at com.enjoy.mj.stackd.A.toString(A.java:23)
at java.lang.String.valueOf(String.java:2979)
at java.lang.StringBuilder.append(StringBuilder.java:131)
at com.enjoy.mj.stackd.B.toString(B.java:23)