Add cached writer. (#3287)

* Add cached writer so we need not call whole method.

* Make writer volatile.

* Make writer to WRITER.

* Rewrite the writeFactory for add the cached writer and keep the unit tests stable.

* Give the default writer.

* Remove volatile.

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/logging/core/WriterFactory.java

@@ -25,23 +25,37 @@ import org.apache.skywalking.apm.agent.core.conf.Config;
 import org.apache.skywalking.apm.agent.core.conf.SnifferConfigInitializer;
 import org.apache.skywalking.apm.util.StringUtil;
 
+/**
+ * @author Alan Lau
+ */
 public class WriterFactory {
+
+    private static IWriter WRITER;
+
     public static IWriter getLogWriter() {
-        if (!useConsole() && SnifferConfigInitializer.isInitCompleted() && AgentPackagePath.isPathFound()) {
-            if (StringUtil.isEmpty(Config.Logging.DIR)) {
-                try {
-                    Config.Logging.DIR = AgentPackagePath.getPath() + "/logs";
-                } catch (AgentPackageNotFoundException e) {
-                    e.printStackTrace();
+
+        switch (Config.Logging.OUTPUT) {
+            case FILE:
+                if (WRITER != null) {
+                    return WRITER;
                 }
-            }
-            return FileWriter.get();
-        } else {
-            return SystemOutWriter.INSTANCE;
-        }
-    }
+                if (SnifferConfigInitializer.isInitCompleted() && AgentPackagePath.isPathFound()) {
+                    if (StringUtil.isEmpty(Config.Logging.DIR)) {
+                        try {
+                            Config.Logging.DIR = AgentPackagePath.getPath() + "/logs";
+                        } catch (AgentPackageNotFoundException e) {
+                            e.printStackTrace();
+                        }
+                    }
+                    WRITER = FileWriter.get();
+                } else {
+                    return SystemOutWriter.INSTANCE;
+                }
+                break;
+            default:
+                return SystemOutWriter.INSTANCE;
 
-    private static boolean useConsole() {
-        return LogOutput.CONSOLE == Config.Logging.OUTPUT;
+        }
+        return WRITER;
     }
 }