Some improvements for Java interoperability

core/src/main/kotlin/ch/dissem/bitmessage/entity/CustomMessage.kt

@@ -68,11 +68,13 @@ open class CustomMessage(val customCommand: String, private val data: ByteArray?
     companion object {
         val COMMAND_ERROR = "ERROR"
 
+        @JvmStatic
         fun read(`in`: InputStream, length: Int): CustomMessage {
             val counter = AccessCounter()
             return CustomMessage(varString(`in`, counter), bytes(`in`, length - counter.length()))
         }
 
+        @JvmStatic
         fun error(message: String): CustomMessage {
             return CustomMessage(COMMAND_ERROR, message.toByteArray(charset("UTF-8")))
         }

extensions/src/main/kotlin/ch/dissem/bitmessage/extensions/pow/ProofOfWorkRequest.kt

@@ -77,6 +77,7 @@ data class ProofOfWorkRequest @JvmOverloads constructor(val sender: BitmessageAd
     }
 
     companion object {
+        @JvmStatic
         fun read(client: BitmessageAddress, `in`: InputStream): ProofOfWorkRequest {
             return ProofOfWorkRequest(
                 client,

repositories/src/main/kotlin/ch/dissem/bitmessage/repository/JdbcHelper.kt

@@ -23,7 +23,7 @@ import java.sql.PreparedStatement
 /**
  * Helper class that does Flyway migration, provides JDBC connections and some helper methods.
  */
-abstract class JdbcHelper protected constructor(protected val config: JdbcConfig) {
+abstract class JdbcHelper protected constructor(@JvmField protected val config: JdbcConfig) {
     companion object {
         @JvmStatic fun writeBlob(ps: PreparedStatement, parameterIndex: Int, data: Streamable?) {
             if (data == null) {