Suppress Default Values from MDC (#120)

The library can add fields from the MDC to each generated log messages.
Certain fields are always created, even if the value is unknown. This changes
avoids those fields in the output. This reduces message size and avoids
unnecessary fields in the target service.
The old behaviour is still available by an extra configuration in the logback.xml
or log4j2.xml.

Signed-off-by: Karsten Schnitter <[email protected]>

Author: KarstenSchnitter

cf-java-logging-support-core/src/main/java/com/sap/hcp/cf/logging/common/converter/DefaultPropertiesConverter.java

@@ -14,11 +14,13 @@
 import com.fasterxml.jackson.jr.ob.JSON;
 import com.fasterxml.jackson.jr.ob.JSONComposer;
 import com.fasterxml.jackson.jr.ob.comp.ObjectComposer;
+import com.sap.hcp.cf.logging.common.Defaults;
 import com.sap.hcp.cf.logging.common.LogContext;
 
 public class DefaultPropertiesConverter {
 
     private final Set<String> exclusions = new HashSet<String>();
+    private boolean sendDefaultValues = false;
 
     public DefaultPropertiesConverter() {
     }
@@ -31,6 +33,10 @@ public void setExclusions(List<String> exclusionList) {
         }
     }
 
+    public void setSendDefaultValues(boolean sendDefaultValues) {
+        this.sendDefaultValues = sendDefaultValues;
+    }
+
     private static class LoggerHolder {
         static final Logger LOG = LoggerFactory.getLogger(LoggerHolder.class.getEnclosingClass());
     }
@@ -46,7 +52,14 @@ public void convert(StringBuilder appendTo, Map<String, String> eventProperties)
                 ObjectComposer<JSONComposer<String>> oc = JSON.std.composeString().startObject();
                 for (Entry<String, String> p: properties.entrySet()) {
                     if (!exclusions.contains(p.getKey())) {
-                        oc.put(p.getKey(), p.getValue());
+                        if (sendDefaultValues) {
+                            oc.put(p.getKey(), p.getValue());
+                        } else {
+                            String defaultValue = getDefaultValue(p.getKey());
+                            if (!defaultValue.equals(p.getValue())) {
+                                oc.put(p.getKey(), p.getValue());
+                            }
+                        }
                     }
                 }
                 String result = oc.end().finish().trim();
@@ -73,4 +86,9 @@ private Map<String, String> mergeContextMaps(Map<String, String> eventMap) {
         }
         return result;
     }
+
+    private String getDefaultValue(String key) {
+        String defaultValue = LogContext.getDefault(key);
+        return defaultValue == null ? Defaults.UNKNOWN : defaultValue;
+    }
 }