XStream - Convert all empty strings on serialization

Question

I want to create a converter that convert all empty strings ("") to null. I created a converter but happens NullPointerException when I register it.

Version 1.4.7

Can someone helpe me, please?

Converter:

public class EmptyConverter extends StringConverter {

     @Override
     public String toString(Object obj) {
         String str = (String) obj;
         if(str != null && str.isEmpty()) {
             return null;
         }
         return super.toString(obj);
     }

}

StackTrace:

java.lang.NullPointerException
    at com.thoughtworks.xstream.io.
xml.PrettyPrintWriter.writeText(PrettyPrintWriter.java:235)
    at com.thoughtworks.xstream.io.xml.PrettyPrintWriter.writeText(PrettyPrintWriter.java:231)
    at com.thoughtworks.xstream.io.xml.PrettyPrintWriter.setValue(PrettyPrintWriter.java:214)
    at com.thoughtworks.xstream.converters.SingleValueConverterWrapper.marshal(SingleValueConverterWrapper.java:45)
    at com.thoughtworks.xstream.core.TreeMarshaller.convert(TreeMarshaller.java:70)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshallField(AbstractReflectionConverter.java:250)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.writeField(AbstractReflectionConverter.java:226)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.<init>(AbstractReflectionConverter.java:189)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doMarshal(AbstractReflectionConverter.java:135)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshal(AbstractReflectionConverter.java:83)
    at com.thoughtworks.xstream.core.TreeMarshaller.convert(TreeMarshaller.java:70)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshallField(AbstractReflectionConverter.java:250)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.writeField(AbstractReflectionConverter.java:226)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.<init>(AbstractReflectionConverter.java:189)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doMarshal(AbstractReflectionConverter.java:135)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshal(AbstractReflectionConverter.java:83)
    at com.thoughtworks.xstream.core.TreeMarshaller.convert(TreeMarshaller.java:70)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshallField(AbstractReflectionConverter.java:250)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.writeField(AbstractReflectionConverter.java:226)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.<init>(AbstractReflectionConverter.java:189)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doMarshal(AbstractReflectionConverter.java:135)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshal(AbstractReflectionConverter.java:83)
    at com.thoughtworks.xstream.core.TreeMarshaller.convert(TreeMarshaller.java:70)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshallField(AbstractReflectionConverter.java:250)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.writeField(AbstractReflectionConverter.java:226)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.<init>(AbstractReflectionConverter.java:189)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doMarshal(AbstractReflectionConverter.java:135)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshal(AbstractReflectionConverter.java:83)
    at com.thoughtworks.xstream.core.TreeMarshaller.convert(TreeMarshaller.java:70)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:43)
    at com.thoughtworks.xstream.converters.collections.AbstractCollectionConverter.writeItem(AbstractCollectionConverter.java:64)
    at com.thoughtworks.xstream.converters.collections.CollectionConverter.marshal(CollectionConverter.java:74)
    at com.thoughtworks.xstream.core.TreeMarshaller.convert(TreeMarshaller.java:70)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshallField(AbstractReflectionConverter.java:250)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.writeField(AbstractReflectionConverter.java:226)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.<init>(AbstractReflectionConverter.java:189)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doMarshal(AbstractReflectionConverter.java:135)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshal(AbstractReflectionConverter.java:83)
    at com.thoughtworks.xstream.core.TreeMarshaller.convert(TreeMarshaller.java:70)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshallField(AbstractReflectionConverter.java:250)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.writeField(AbstractReflectionConverter.java:226)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$2.<init>(AbstractReflectionConverter.java:189)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doMarshal(AbstractReflectionConverter.java:135)
    at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.marshal(AbstractReflectionConverter.java:83)
    at com.thoughtworks.xstream.core.TreeMarshaller.convert(TreeMarshaller.java:70)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:43)
    at com.thoughtworks.xstream.core.TreeMarshaller.start(TreeMarshaller.java:82)
    at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.marshal(AbstractTreeMarshallingStrategy.java:37)
    at com.thoughtworks.xstream.XStream.marshal(XStream.java:1022)
    at com.thoughtworks.xstream.XStream.marshal(XStream.java:1011)
    at com.thoughtworks.xstream.XStream.toXML(XStream.java:984)
    at com.thoughtworks.xstream.XStream.toXML(XStream.java:971)
    at br.com.flexait.nfse.builder.Nfse.asXML(Nfse.java:43)
    at br.com.flexait.nfse.builder.NfseTest.shouldSetNullIfStringEmpty(NfseTest.java:187)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

Answer 1

XStream respects Java's own serilization mechanism, so you can use writeReplace for that. This method get invoked before the object get serialized. The object can simply replace it before.

    public class NullString {
        public static void main(String[] args) {
            final Foo foo = new Foo();
            foo.bar = "hello";
            foo.baz = "";

            final XStream xstream = new XStream();
            xstream.toXML(foo, System.out);
        }
    }

    class Foo {
        String bar;
        String baz;

        private Object writeReplace() throws ObjectStreamException {
            final Foo replacement = new Foo();
            //@formatter:off
            replacement.bar = (this.bar == null || this.bar.isEmpty()) ? null : this.bar;
            replacement.baz = (this.baz == null || this.baz.isEmpty()) ? null : this.baz;
            //@formatter:on

            return replacement;
        }
    }

Prints out

    <org.example.Foo>
      <bar>hello</bar>
    </org.example.Foo>

instead of

    <org.example.Foo>
      <bar>hello</bar>
      <baz></baz>
    </org.example.Foo>