Created StringBuilders thread local holder and using thread local StringBuilder in Log4jAdapterLabelPrinter that can be shared between many threads.

log4j2-appender/src/main/java/pl/tkowalcz/tjahzi/log4j2/labels/Log4jAdapterLabelPrinter.java

@@ -4,6 +4,7 @@ import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.pattern.LiteralPatternConverter;
 import org.apache.logging.log4j.core.pattern.LogEventPatternConverter;
 import org.apache.logging.log4j.core.pattern.PatternFormatter;
+import pl.tkowalcz.tjahzi.log4j2.utils.StringBuilders;
 
 import java.util.List;
 import java.util.function.Consumer;
@@ -11,7 +12,6 @@ import java.util.function.Consumer;
 @SuppressWarnings("ForLoopReplaceableByForEach")
 public class Log4jAdapterLabelPrinter implements LabelPrinter {
 
-    private final StringBuilder outputBuffer = new StringBuilder();
     private final List<PatternFormatter> formatters;
 
     public Log4jAdapterLabelPrinter(List<PatternFormatter> formatters) {

log4j2-appender/src/main/java/pl/tkowalcz/tjahzi/log4j2/utils/StringBuilders.java

@@ -0,0 +1,13 @@
+package pl.tkowalcz.tjahzi.log4j2.utils;
+
+public class StringBuilders {
+
+    private static final ThreadLocal<StringBuilder> THREAD_LOCAL = ThreadLocal.withInitial(StringBuilder::new);
+
+    public static StringBuilder threadLocal() {
+        StringBuilder result = THREAD_LOCAL.get();
+        result.setLength(0);
+
+        return result;
+    }
+}

log4j2-appender/src/test/java/pl/tkowalcz/tjahzi/log4j2/utils/StringBuildersTest.java

@@ -0,0 +1,55 @@
+package pl.tkowalcz.tjahzi.log4j2.utils;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class StringBuildersTest {
+
+    @Test
+    void shouldClearBufferBeforeUse() {
+        // Given
+        StringBuilder stringBuilder = StringBuilders.threadLocal();
+        stringBuilder.append("abc");
+
+        // When
+        StringBuilder actual = StringBuilders.threadLocal();
+
+        // Then
+        assertThat(actual.toString()).isEmpty();
+    }
+
+    @Test
+    void shouldReturnSameBufferInsideSameThread() {
+        // When
+        StringBuilder stringBuilder1 = StringBuilders.threadLocal();
+        StringBuilder stringBuilder2 = StringBuilders.threadLocal();
+
+        // Then
+        assertThat(stringBuilder1).isSameAs(stringBuilder2);
+    }
+
+    @Test
+    void shouldReturnDifferentBufferInsideDifferentThreads() throws InterruptedException {
+        // Given
+        AtomicReference<StringBuilder> captor1 = new AtomicReference<>();
+        AtomicReference<StringBuilder> captor2 = new AtomicReference<>();
+
+        // When
+        Thread thread1 = new Thread(() -> captor1.set(StringBuilders.threadLocal()));
+        thread1.start();
+        thread1.join();
+
+        Thread thread2 = new Thread(() -> captor2.set(StringBuilders.threadLocal()));
+        thread2.start();
+        thread2.join();
+
+        // Then
+        assertThat(captor1.get()).isNotNull();
+        assertThat(captor2.get()).isNotNull();
+
+        assertThat(captor1.get()).isNotSameAs(captor2.get());
+    }
+}