DbFullException: test max size when opening, clarify in docs

In response to #1199

Author: greenrobot-team

objectbox-java/src/main/java/io/objectbox/BoxStoreBuilder.java

@@ -369,13 +369,13 @@ BoxStoreBuilder modelUpdate(ModelUpdate modelUpdate) {
 
     /**
      * Sets the maximum size the database file can grow to.
-     * When applying a transaction (e.g. putting an object) would exceed it a {@link DbFullException} is thrown.
      * <p>
-     * By default, this is 1 GB, which should be sufficient for most applications.
-     * In general, a maximum size prevents the database from growing indefinitely when something goes wrong
-     * (for example data is put in an infinite loop).
+     * The Store will throw when the file size is about to be exceeded, see {@link DbFullException} for details.
      * <p>
-     * This value can be changed, so increased or also decreased, each time when opening a store.
+     * By default, this is 1 GB, which should be sufficient for most applications. In general, a maximum size prevents
+     * the database from growing indefinitely when something goes wrong (for example data is put in an infinite loop).
+     * <p>
+     * This can be set to a value different, so higher or also lower, from when last building the Store.
      */
     public BoxStoreBuilder maxSizeInKByte(long maxSizeInKByte) {
         if (maxSizeInKByte <= maxDataSizeInKByte) {

objectbox-java/src/main/java/io/objectbox/exception/DbFullException.java

@@ -21,7 +21,7 @@
  * {@link io.objectbox.BoxStoreBuilder#maxSizeInKByte(long) maxSizeInKByte} configured for the Store.
  * <p>
  * This can occur for operations like when an Object is {@link io.objectbox.Box#put(Object) put}, at the point when the
- * (internal) transaction is committed. Or when the Store is opened with a max size smaller than the existing database.
+ * (internal) transaction is committed. Or when the Store is opened with a max size too small for the existing database.
  */
 public class DbFullException extends DbException {
     public DbFullException(String message) {

tests/objectbox-java-test/src/test/java/io/objectbox/BoxStoreBuilderTest.java

@@ -262,6 +262,14 @@ public void maxFileSize() {
         assumeFalse(IN_MEMORY); // no max size support for in-memory
 
         builder = createBoxStoreBuilder(null);
+        // The empty data.mdb file is around 12 KB, but creating will fail also if slightly above that
+        builder.maxSizeInKByte(15);
+        DbFullException couldNotPut = assertThrows(
+                DbFullException.class,
+                () -> builder.build()
+        );
+        assertEquals("Could not put", couldNotPut.getMessage());
+
         builder.maxSizeInKByte(30); // Empty file is around 12 KB, object below adds about 8 KB each.
         store = builder.build();
         putTestEntity(LONG_STRING, 1);