Features/add fields by pojo (influxdata#519)

* Map Object to Point

* Adding fields to builder by passing a POJO

* Removing annotation support on accessors

* Removing unused private method

* Adding documentation

Author: mirza99

README.md

@@ -263,6 +263,22 @@ InfluxDBResultMapper resultMapper = new InfluxDBResultMapper(); // thread-safe -
 List<Cpu> cpuList = resultMapper.toPOJO(queryResult, Cpu.class);
 ```
 
+#### Writing using POJO
+The same way we use `annotations` to transform data to POJO, we can write data as POJO.
+Having the same POJO class Cpu
+
+```java
+String dbName = "myTimeseries";
+String rpName = "aRetentionPolicy";
+// Cpu has annotations @Measurement and @Column
+Cpu cpu = new Cpu();
+// ... setting data
+
+Point point = Point.measurementByPOJO(cpu.getClass()).addFieldsFromPOJO(cpu).build();
+
+influxDB.write(dbName, rpName, point);
+```
+
 #### QueryResult mapper limitations
 
 * If your InfluxDB query contains multiple SELECT clauses, you will have to call InfluxResultMapper#toPOJO() multiple times to map every measurement returned by QueryResult to the respective POJO;

src/main/java/org/influxdb/BuilderException.java

@@ -0,0 +1,18 @@
+package org.influxdb;
+
+/**
+ * Class for exceptions when using Point Builder.
+ *
+ * @author mirza99
+ */
+public class BuilderException extends RuntimeException {
+
+  /**
+   * Generated serial version UID.
+   */
+  private static final long serialVersionUID = 4178882805281378918L;
+
+  public BuilderException(final String message) {
+    super(message);
+  }
+}

src/main/java/org/influxdb/dto/Point.java

@@ -1,5 +1,7 @@
 package org.influxdb.dto;
 
+import java.lang.annotation.Annotation;
+import java.lang.reflect.Field;
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.text.NumberFormat;
@@ -9,6 +11,9 @@
 import java.util.Objects;
 import java.util.TreeMap;
 import java.util.concurrent.TimeUnit;
+import org.influxdb.BuilderException;
+import org.influxdb.annotation.Column;
+import org.influxdb.annotation.Measurement;
 import org.influxdb.impl.Preconditions;
 
 /**
@@ -52,6 +57,28 @@ public static Builder measurement(final String measurement) {
     return new Builder(measurement);
   }
 
+  /**
+   * Create a new Point Build build to create a new Point in a fluent manner from a POJO.
+   *
+   * @param clazz Class of the POJO
+   * @return the Builder instance
+   */
+
+  public static Builder measurementByPOJO(final Class<?> clazz) {
+    Objects.requireNonNull(clazz, "clazz");
+    throwExceptionIfMissingAnnotation(clazz, Measurement.class);
+    String measurementName = findMeasurementName(clazz);
+    return new Builder(measurementName);
+  }
+
+  private static void throwExceptionIfMissingAnnotation(final Class<?> clazz,
+      final Class<? extends Annotation> expectedClass) {
+    if (!clazz.isAnnotationPresent(expectedClass)) {
+      throw new IllegalArgumentException("Class " + clazz.getName() + " is not annotated with @"
+          + Measurement.class.getSimpleName());
+    }
+  }
+
   /**
    * Builder for a new Point.
    *
@@ -195,6 +222,56 @@ public boolean hasFields() {
       return !fields.isEmpty();
     }
 
+    /**
+     * Adds field map from object by reflection using {@link org.influxdb.annotation.Column}
+     * annotation.
+     *
+     * @param pojo POJO Object with annotation {@link org.influxdb.annotation.Column} on fields
+     * @return the Builder instance
+     */
+    public Builder addFieldsFromPOJO(final Object pojo) {
+
+      Class<? extends Object> clazz = pojo.getClass();
+
+      for (Field field : clazz.getDeclaredFields()) {
+
+        Column column = field.getAnnotation(Column.class);
+
+        if (column == null) {
+          continue;
+        }
+
+        field.setAccessible(true);
+        String fieldName = column.name();
+        addFieldByAttribute(pojo, field, column, fieldName);
+      }
+
+      if (this.fields.isEmpty()) {
+        throw new BuilderException("Class " + pojo.getClass().getName()
+            + " has no @" + Column.class.getSimpleName() + " annotation");
+      }
+
+      return this;
+    }
+
+    private void addFieldByAttribute(final Object pojo, final Field field, final Column column,
+        final String fieldName) {
+      try {
+        Object fieldValue = field.get(pojo);
+
+        if (column.tag()) {
+          this.tags.put(fieldName, (String) fieldValue);
+        } else {
+          this.fields.put(fieldName, fieldValue);
+        }
+
+      } catch (IllegalArgumentException | IllegalAccessException e) {
+        // Can not happen since we use metadata got from the object
+        throw new BuilderException(
+            "Field " + fieldName + " could not found on class " + pojo.getClass().getSimpleName());
+      }
+    }
+
     /**
      * Create a new Point.
      *
@@ -254,6 +331,13 @@ void setPrecision(final TimeUnit precision) {
     this.precision = precision;
   }
 
+  /**
+   * @return the fields
+   */
+  Map<String, Object> getFields() {
+    return this.fields;
+  }
+
   /**
    * @param fields
    *            the fields to set
@@ -418,4 +502,8 @@ private void formatedTime(final StringBuilder sb, final TimeUnit precision) {
     }
     sb.append(" ").append(precision.convert(this.time, this.precision));
   }
+
+  private static String findMeasurementName(final Class<?> clazz) {
+    return clazz.getAnnotation(Measurement.class).name();
+  }
 }