Painless Syntax

Painless supports all of Java’s types, including array types, but adds some additional built-in types.

The dynamic type def serves as a placeholder for any other type. It adopts the behavior of whatever runtime type it represents.

String constants can be declared with single quotes, to avoid escaping horrors with JSON:

def mystring = 'foo';

Arrays can be subscripted starting from 0 for traditional array access or with negative numbers to starting from the back of the array. So the following returns 2.

int[] x = new int[5];
x[0]++;
x[-5]++;
return x[0];

Lists can be created explicitly (e.g. new ArrayList()) or initialized similar to Groovy:

def list = [1,2,3];

Lists can also be accessed similar to arrays. They support .length and subscripts, including negative subscripts to read from the back of the list:

def list = [1,2,3];
list[-1] = 5
return list[0]

Maps can be created explicitly (e.g. new HashMap()) or initialized similar to Groovy:

def person = ['name': 'Joe', 'age': 63];

Map keys can also be accessed as properties.

def person = ['name': 'Joe', 'age': 63];
person.retired = true;
return person.name

Map keys can also be accessed via subscript (for keys containing special characters):

return map['something-absurd!']

Regular expression constants are directly supported:

Pattern p = /[aeiou]/

Patterns can only be created via this mechanism. This ensures fast performance, regular expressions are always constants and compiled efficiently a single time.

You can define flags on patterns in Painless by adding characters after the trailing / like /foo/i or /foo \w #comment/iUx. Painless exposes all the flags from Java’s Pattern class using these characters:

Like lots of languages, Painless uses . to reference fields and call methods:

String foo = 'foo';
TypeWithGetterOrPublicField bar = new TypeWithGetterOrPublicField()
return foo.length() + bar.x

Like Groovy, Painless uses ?. to perform null-safe references, with the result being null if the left hand side is null:

String foo = null;
return foo?.length()  // Returns null

Unlike Groovy, Painless doesn’t support writing to null values with this operator:

TypeWithSetterOrPublicField foo = null;
foo?.x = 'bar'  // Compile error

All of Java’s operators are supported with the same precedence, promotion, and semantics.

There are only a few minor differences and add-ons:

The ?: (aka Elvis) operator coalesces null values. So x ?: 0 is 0 if x is null and whatever value x has otherwise. It is a convenient way to write default values like doc['x'].value ?: 0 which is 0 if x is not in the document being processed. It can also work with null safe dereferences to efficiently handle null in chains. For example, doc['foo.keyword'].value?.length() ?: 0 is 0 if the document being processed doesn’t have a foo.keyword field but is the length of that field if it does. Lastly, ?: is lazy so the right hand side is not evaluated at all if the left hand side isn’t null.

Java’s control flow statements are supported, with the exception of the switch statement.

In addition to Java’s enhanced for loop, the for in syntax from groovy can also be used:

for (item : list) {
  ...
}

Functions can be declared at the beginning of the script, for example:

boolean isNegative(def x) { x < 0 }
...
if (isNegative(someVar)) {
  ...
}

Lambda expressions and method references work the same as Java’s.

list.removeIf(item -> item == 2);
list.removeIf((int item) -> item == 2);
list.removeIf((int item) -> { item == 2 });
list.sort((x, y) -> x - y);
list.sort(Integer::compare);

Method references to functions within the script can be accomplished using this, e.g. list.sort(this::mycompare).