Make it possible to pretty print throwables as array of strings.

README.md

@@ -1568,6 +1568,18 @@ For example:
 See the [net.logstash.logback.decorate](/src/main/java/net/logstash/logback/decorate) package
 and sub-packages for other decorators.
 
+If you prefer pretty printing for easier interactive viewing of (error) logs, you
+may also prefer to output throwable as json array of strings where each string
+is a stacktrace line:
+
+```xml
+<encoder class="net.logstash.logback.encoder.LogstashEncoder">
+	<jsonGeneratorDecorator class="net.logstash.logback.decorate.PrettyPrintingJsonGeneratorDecorator">
+		<indentArraysWithNewLine>true</indentArraysWithNewLine>
+	<outputThrowableAsArray>true</outputThrowableAsArray>
+</encoder>
+```
+
 ### Registering Jackson Modules
 
 By default, Jackson modules are dynamically registered via
@@ -2766,6 +2778,7 @@ The provider name is the xml element name to use when configuring. Each provider
         <ul>
           <li><tt>fieldName</tt> - Output field name (<tt>stack_trace</tt>)</li>
           <li><tt>throwableConverter</tt> - The <tt>ThrowableHandlingConverter</tt> to use to format the stacktrace (<tt>stack_trace</tt>)</li>
+          <li><tt>outputThrowableAsArray</tt> - Output throwable as json array of strings where each string is a stacktrace line</li>
         </ul>
       </td>
     </tr>

src/main/java/net/logstash/logback/LogstashFormatter.java

@@ -396,6 +396,14 @@ public void setThrowableConverter(ThrowableHandlingConverter throwableConverter)
         this.stackTraceProvider.setThrowableConverter(throwableConverter);
     }
 
+    public boolean isOutputThrowableAsArray() {
+        return this.stackTraceProvider.isOutputThrowableAsArray();
+    }
+
+    public void setOutputThrowableAsArray(boolean outputAsArray) {
+        this.stackTraceProvider.setOutputThrowableAsArray(outputAsArray);
+    }
+
     public String getVersion() {
         return this.versionProvider.getVersion();
     }

src/main/java/net/logstash/logback/composite/loggingevent/StackTraceJsonProvider.java

@@ -32,6 +32,12 @@ public class StackTraceJsonProvider extends AbstractFieldJsonProvider<ILoggingEv
 
     public static final String FIELD_STACK_TRACE = "stack_trace";
 
+    /**
+     * If true, stacktrace will be output as a json array of strings split by newlines
+     * If else, stacktrace will be output as a json string
+     */
+    private boolean outputThrowableAsArray = false;
+
     /**
      * Used to format throwables as Strings.
      *
@@ -47,6 +53,14 @@ public StackTraceJsonProvider() {
         setFieldName(FIELD_STACK_TRACE);
     }
 
+    public boolean isOutputThrowableAsArray() {
+        return outputThrowableAsArray;
+    }
+
+    public void setOutputThrowableAsArray(boolean outputThrowableAsArray) {
+        this.outputThrowableAsArray = outputThrowableAsArray;
+    }
+
     @Override
     public void start() {
         this.throwableConverter.start();
@@ -62,8 +76,15 @@ public void stop() {
     @Override
     public void writeTo(JsonGenerator generator, ILoggingEvent event) throws IOException {
         IThrowableProxy throwableProxy = event.getThrowableProxy();
-        if (throwableProxy != null) {
-            JsonWritingUtils.writeStringField(generator, getFieldName(), throwableConverter.convert(event));
+        if (throwableProxy == null) {
+            return;
+        }
+        String stacktrace = throwableConverter.convert(event);
+        if (outputThrowableAsArray) {
+            String[] lines = stacktrace.split("\n");
+            JsonWritingUtils.writeStringArrayField(generator, getFieldName(), lines);
+        } else {
+            JsonWritingUtils.writeStringField(generator, getFieldName(), stacktrace);
         }
     }
 

src/main/java/net/logstash/logback/decorate/PrettyPrintingJsonGeneratorDecorator.java

@@ -17,6 +17,7 @@
 
 import ch.qos.logback.core.CoreConstants;
 import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.core.util.DefaultIndenter;
 import com.fasterxml.jackson.core.util.DefaultPrettyPrinter;
 
 /**
@@ -62,4 +63,12 @@ public void setSpacesInObjectEntries(boolean spacesInObjectEntries) {
             prettyPrinter = prettyPrinter.withoutSpacesInObjectEntries();
         }
     }
+
+    public void setIndentArraysWithNewLine(boolean indentArraysWithNewLine) {
+        if (indentArraysWithNewLine) {
+            prettyPrinter.indentArraysWith(DefaultIndenter.SYSTEM_LINEFEED_INSTANCE);
+        } else {
+            prettyPrinter.indentArraysWith(null);
+        }
+    }
 }

src/main/java/net/logstash/logback/encoder/LogstashEncoder.java

@@ -203,6 +203,14 @@ public void setThrowableConverter(ThrowableHandlingConverter throwableConverter)
         getFormatter().setThrowableConverter(throwableConverter);
     }
 
+    public boolean isOutputThrowableAsArray() {
+        return getFormatter().isOutputThrowableAsArray();
+    }
+
+    public void setOutputThrowableAsArray(boolean outputAsArray) {
+        this.getFormatter().setOutputThrowableAsArray(outputAsArray);
+    }
+
     public String getTimeZone() {
         return getFormatter().getTimeZone();
     }

src/test/java/net/logstash/logback/composite/loggingevent/StackTraceJsonProviderTest.java

@@ -15,6 +15,8 @@
  */
 package net.logstash.logback.composite.loggingevent;
 
+import static org.mockito.Mockito.anyString;
+import static org.mockito.Mockito.atLeastOnce;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
@@ -26,6 +28,7 @@
 import ch.qos.logback.classic.spi.ILoggingEvent;
 import ch.qos.logback.classic.spi.IThrowableProxy;
 import com.fasterxml.jackson.core.JsonGenerator;
+import org.assertj.core.util.Throwables;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -90,4 +93,21 @@ public void testFieldNames() throws IOException {
         verify(generator).writeStringField("newFieldName", "stack");
     }
 
+    @Test
+    public void testOutputAsArray() throws IOException {
+        String stacktrace = Throwables.getStackTrace(new RuntimeException("testing exception handling"));
+        when(converter.convert(event)).thenReturn(stacktrace);
+
+        provider.setOutputThrowableAsArray(true);
+
+        when(event.getThrowableProxy()).thenReturn(ThrowableProxy);
+
+        provider.writeTo(generator, event);
+
+        verify(generator).writeArrayFieldStart("stack_trace");
+        verify(generator).writeString("java.lang.RuntimeException: testing exception handling");
+        verify(generator, atLeastOnce()).writeString(anyString());
+        verify(generator).writeEndArray();
+    }
+
 }

src/test/java/net/logstash/logback/decorate/PrettyPrintingJsonGeneratorDecoratorTest.java

@@ -19,6 +19,7 @@
 
 import java.io.IOException;
 import java.io.StringWriter;
+import java.util.Arrays;
 import java.util.Collections;
 
 import com.fasterxml.jackson.core.JsonGenerator;
@@ -94,4 +95,29 @@ void noSpacesInObjectEntries() throws IOException {
         writer.flush();
         assertThat(writer.toString()).isEqualTo("{\n  \"key1\":\"value1\"\n}{\n  \"key2\":\"value2\"\n}");
     }
+
+    @Test
+    void arrayElementsOnNewLine() throws IOException {
+        PrettyPrintingJsonGeneratorDecorator decorator = new PrettyPrintingJsonGeneratorDecorator();
+        decorator.setIndentArraysWithNewLine(true);
+
+        StringWriter writer = new StringWriter();
+        ObjectMapper objectMapper = new ObjectMapper();
+        JsonGenerator generator = decorator.decorate(objectMapper.createGenerator(writer));
+
+        generator.writeObject(Collections.singletonMap("key1", Arrays.asList("RuntimeException: foobar",
+                "\tat com.example.Foobar")));
+        generator.writeObject(Collections.singletonMap("key2", "value2"));
+
+        generator.flush();
+        writer.flush();
+        assertThat(writer.toString()).isEqualTo("{\n" +
+                "  \"key1\" : [\n" +
+                "    \"RuntimeException: foobar\",\n" +
+                "    \"\\tat com.example.Foobar\"\n" +
+                "  ]\n" +
+                "}{\n" +
+                "  \"key2\" : \"value2\"\n" +
+                "}");
+    }
 }