Android support annotation “should have @retention(retentionpolicy.source)”

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Java

Android

I am using the @Retention and @StringDef annotations for some of my methods in a library and I face a strange warning, I want to understand.

What I have tried:

In a static class, SIP, I use this annotation:

public static final String CODEC_SPEEX_16K  = "speex/16000/1";
public static final String CODEC_SPEEX_8K   = "speex/8000/1";
public static final String CODEC_SPEEX_32K  = "speex/32000/1";
public static final String CODEC_ILBC_8K    = "iLBC/8000/1";
public static final String CODEC_GSM_8K     = "GSM/8000/1";
public static final String CODEC_PCMU_8K    = "PCMU/8000/1";
public static final String CODEC_PCMA_8K    = "PCMA/8000/1";
public static final String CODEC_G722_16K   = "G722/16000/1";

@Retention(RetentionPolicy.CLASS)
@StringDef({
    CODEC_SPEEX_16K,
    CODEC_SPEEX_8K,
    CODEC_SPEEX_32K,
    CODEC_ILBC_8K,
    CODEC_GSM_8K,
    CODEC_PCMU_8K,
    CODEC_PCMA_8K,
    CODEC_G722_16K
    })
public @interface CodecName {}

which compiles fine, without any warnings.

In the static class Tools I use this annotation:

public static final String RES_TYPE_STRING = "string";
public static final String RES_TYPE_DRAWABLE = "drawable";
public static final String RES_TYPE_LAYOUT = "layout";
public static final String RES_TYPE_VIEW = "id";
public static final String RES_TYPE_DIMEN = "dimen";
public static final String RES_TYPE_COLOR = "color";
public static final String RES_TYPE_ANIM = "anim";
public static final String RES_TYPE_MIPMAP = "mipmap";

@Retention(RetentionPolicy.CLASS)
@StringDef({
    RES_TYPE_STRING,
    RES_TYPE_DRAWABLE,
    RES_TYPE_LAYOUT,
    RES_TYPE_DIMEN,
    RES_TYPE_COLOR,
    RES_TYPE_ANIM,
    RES_TYPE_VIEW,
    RES_TYPE_MIPMAP
})
public @interface ResourceType {
}

and I get the warning:

The typedef annotation ....toolbox.Tools.ResourceType should have @Retention(RetentionPolicy.SOURCE)

It seems to be uncritical, everything works fine. But can someone please explain me, why Annotation#1 does not get a warning and #2 gets a warning? Both are built identical, both only used in static context. From my point of view, both are the same.