java.lang.ClassCastException while implementing writeGcs() as given example in https://github.com/spring-cloud/spring-cloud-gcp/blob/master/spring-cloud-gcp-examples/spring-cloud-gcp-storage-resource-example/src/main/java/com/example/WebController.java

[pankajnd20]

error description :
java.lang.ClassCastException: org.springframework.web.context.support.ServletContextResource cannot be cast to org.springframework.core.io.WritableResource

error line :
try (OutputStream os = ((WritableResource) this.gcsFile).getOutputStream())

full code :

import org.springframework.beans.factory.annotation.Value;
import org.springframework.core.io.Resource;
import org.springframework.core.io.WritableResource;

@value("gs://mybucketname/my-file.txt")
private Resource gcsFile;

public String readGcsFile() throws IOException {
return StreamUtils.copyToString(
this.gcsFile.getInputStream(),
Charset.defaultCharset()) + "\n";
}

String writeGcs(String data) throws IOException {
    try (OutputStream os = ((WritableResource) this.gcsFile).getOutputStream()) {
        os.write(data.getBytes());
    }
    return "file was updated\n";
}

why it's casting super class to sub class in the example ?

[meltsufin]

It looks like you're missing the Google Cloud Storage Spring Boot Starter dependency and the gs://mybucketname/my-file.txt URI is being interpreted as a ServletContextResource rather than the GoogleStorageResource. Hence, you're unable to cast to WritableResource, which is not implemented by ServletContextResource.

Does your pom.xml contain this dependency?

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-gcp-starter-storage</artifactId>
</dependency>

The reason we have to cast is because @value currently only works with the Resource types, not WritableResource. See this issue on the JIRA tracker: https://jira.spring.io/browse/SPR-10656.

[pankajnd20]

Already added the mentioned dependency in pom.xml. Also checked library exist in project or not .
Though getting this error.

[meltsufin]

Can you share a complete example that reproduces this? Maybe change our example to match your setup.

[saturnism]

@pankajnd20 could i trouble you to check my example here?
https://github.com/saturnism/spring-cloud-gcp-guestbook/tree/master/6-storage/guestbook-frontend

Also, some lab instructions I used:
https://docs.google.com/document/d/14NRxngEhkh1gPTyTS-V51XEHrGMhJKnbzw5uSs6oFaI/edit#heading=h.4civimycga8m

Cheers,

[joaoandremartins]

Also, in your example you don't seem to be writing to the resource.

[joaoandremartins]

Please reopen if you are still seeing this issue.

[saurabhguptasg]

Hi all, this issue is still occurring
with the storage and starter dependencies both included, the resource gets created as a ServletContext resource and throws a cast exception for WritableResource when invoking:
OutputStream storageOutputStream = ((WritableResource)storageObject).getOutputStream();
the sample referenced in this bug has now moved to:
https://github.com/spring-cloud/spring-cloud-gcp/blob/master/spring-cloud-gcp-samples/spring-cloud-gcp-storage-resource-sample/src/main/java/com/example/WebController.java

[joaoandremartins]

@saurabhguptasg when you run the code sample, is the resource created as ServletContextResource?

[sayi65]

i get the same issue.
I wrote the same source as the example

error:
org.springframework.web.context.support.ServletContextResource cannot be cast to org.springframework.core.io.WritableResource

[joaoandremartins]

There is a working Storage code sample in https://github.com/spring-cloud/spring-cloud-gcp/tree/master/spring-cloud-gcp-samples/spring-cloud-gcp-storage-resource-sample.

Would you mind trying it and then adapting to reproduce the issue you're seeing? Please let us know when you were able to do so.

[sayi65]

@joaoandremartins
OK thank you for your comment