aosp12/external/escapevelocity

EscapeVelocity summary

EscapeVelocity is a templating engine that can be used from Java. It is a reimplementation of a subset of functionality from Apache Velocity.

This is not an official Google product.

For a fuller explanation of Velocity's functioning, see its User Guide

If EscapeVelocity successfully produces a result from a template evaluation, that result should be the exact same string that Velocity produces. If not, that is a bug.

EscapeVelocity has no facilities for HTML escaping and it is not appropriate for producing HTML output that might include portions of untrusted input.

Motivation

Velocity has a convenient templating language. It is easy to read, and it has widespread support from tools such as editors and coding websites. However, using Velocity can prove difficult. Its use to generate Java code in the AutoValue annotation processor required many workarounds. The way it dynamically loads classes as part of its standard operation makes it hard to shade it, which in the case of AutoValue led to interference if Velocity was used elsewhere in a project.

EscapeVelocity has a simple API that does not involve any class-loading or other sources of problems. It and its dependencies can be shaded with no difficulty.

Loading a template

The entry point for EscapeVelocity is the Template class. To obtain an instance, use Template.from(Reader). If a template is stored in a file, that file conventionally has the suffix .vm (for Velocity Macros). But since the argument is a Reader, you can also load a template directly from a Java string, using StringReader.

Here's how you might make a Template instance from a template file that is packaged as a resource in the same package as the calling class:

InputStream in = getClass().getResourceAsStream("foo.vm");
if (in == null) {
  throw new IllegalArgumentException("Could not find resource foo.vm");
}
Reader reader = new BufferedReader(new InputStreamReader(in));
Template template = Template.parseFrom(reader);

Expanding a template

Once you have a Template object, you can use it to produce a string where the variables in the template are given the values you provide. You can do this any number of times, specifying the same or different values each time.

Suppose you have this template:

The $language word for $original is $translated.

You might write this code:

Map<String, String> vars = new HashMap<>();
vars.put("language", "French");
vars.put("original", "toe");
vars.put("translated", "orteil");
String result = template.evaluate(vars);

The result string would then be: The French word for toe is orteil.

Comments

The characters ## introduce a comment. Characters from ## up to and including the following newline are omitted from the template. This template has comments:

Line 1 ## with a comment
Line 2

It is the same as this template:

Line 1 Line 2

References

EscapeVelocity supports most of the reference types described in the Velocity User Guide

Variables

A variable has an ASCII name that starts with a letter (a-z or A-Z) and where any other characters are also letters or digits or hyphens (-) or underscores (_). A variable reference can be written as $foo or as ${foo}. The value of a variable can be of any Java type. If the value v of variable foo is not a String then the result of $foo in a template will be String.valueOf(v). Variables must be defined before they are referenced; otherwise an EvaluationException will be thrown.

Variable names are case-sensitive: $foo is not the same variable as $Foo or $FOO.

Initially the values of variables come from the Map that is passed to Template.evaluate. Those values can be changed, and new ones defined, using the #set directive in the template:

#set ($foo = "bar")

Setting a variable affects later references to it in the template, but has no effect on the Map that was passed in or on later template evaluations.

Properties

If a reference looks like $purchase.Total then the value of the $purchase variable must be a Java object that has a public method getTotal() or gettotal(), or a method called isTotal() or istotal() that returns boolean. The result of $purchase.Total is then the result of calling that method on the $purchase object.

If you want to have a period (.) after a variable reference without it being a property reference, you can use braces like this: ${purchase}.Total. If, after a property reference, you have a further period, you can put braces around the reference like this: ${purchase.Total}.nonProperty.

Methods

If a reference looks like $purchase.addItem("scones", 23) then the value of the $purchase variable must be a Java object that has a public method addItem with two parameters that match the given values. Unlike Velocity, EscapeVelocity requires that there be exactly one such method. It is OK if there are other addItem methods provided they are not compatible with the arguments provided.

Properties are in fact a special case of methods: instead of writing $purchase.Total you could write $purchase.getTotal(). Braces can be used to make the method invocation explicit (${purchase.getTotal()}) or to prevent method invocation (${purchase}.getTotal()).

Indexing

If a reference looks like $indexme[$i] then the value of the $indexme variable must be a Java object that has a public get method that takes one argument that is compatible with the index. For example, $indexme might be a List and $i might be an integer. Then the reference would be the result of List.get(int) for that list and that integer. Or, $indexme might be a Map, and the reference would be the result of Map.get(Object) for the object $i. In general, $indexme[$i] is equivalent to $indexme.get($i).

Unlike Velocity, EscapeVelocity does not allow $indexme to be a Java array.

Undefined references

If a variable has not been given a value, either by being in the initial Map argument or by being set in the template, then referencing it will provoke an EvaluationException. There is a special case for #if: if you write #if ($var) then it is allowed for $var not to be defined, and it is treated as false.

Setting properties and indexes: not supported

Unlke Velocity, EscapeVelocity does not allow #set assignments with properties or indexes:

#set ($data.User = "jon")        ## Allowed in Velocity but not in EscapeVelocity
#set ($map["apple"] = "orange")  ## Allowed in Velocity but not in EscapeVelocity

Expressions

In certain contexts, such as the #set directive we have just seen or certain other directives, EscapeVelocity can evaluate expressions. An expression can be any of these:

• A reference, of the kind we have just seen. The value is the value of the reference.
• A string literal enclosed in double quotes, like "this". A string literal must appear on one line. EscapeVelocity does not support the characters $ or \\ in a string literal.
• An integer literal such as 23 or -100. EscapeVelocity does not support floating-point literals.
• A Boolean literal, true or false.
• Simpler expressions joined together with operators that have the same meaning as in Java: !, ==, !=, <, <=, >, >=, &&, ||, +, -, *, /, %. The operators have the same precedence as in Java.
• A simpler expression in parentheses, for example (2 + 3).

Velocity supports string literals with single quotes, like 'this' and also references within strings, like "a $reference in a string", but EscapeVelocity does not.

Directives

A directive is introduced by a # character followed by a word. We have already seen the #set directive, which sets the value of a variable. The other directives are listed below.

Directives can be spelled with or without braces, so #set or #{set}.

#if/#elseif/#else

The #if directive selects parts of the template according as a condition is true or false. The simplest case looks like this:

#if ($condition) yes #end

This evaluates to the string yes if the variable $condition is defined and has a true value, and to the empty string otherwise. It is allowed for $condition not to be defined in this case, and then it is treated as false.

The expression in #if (here $condition) is considered true if its value is not null and not equal to the Boolean value false.

An #if directive can also have an #else part, for example:

#if ($condition) yes #else no #end

This evaluates to the string yes if the condition is true or the string no if it is not.

An #if directive can have any number of #elseif parts. For example:

#if ($i == 0) zero #elseif ($i == 1) one #elseif ($i == 2) two #else many #end

#foreach

The #foreach directive repeats a part of the template once for each value in a list.

#foreach ($product in $allProducts)
  ${product}!
#end

This will produce one line for each value in the $allProducts variable. The value of $allProducts can be a Java Iterable, such as a List or Set; or it can be an object array; or it can be a Java Map. When it is a Map the #foreach directive loops over every value in the Map.

If $allProducts is a List containing the strings oranges and lemons then the result of the #foreach would be this:

  oranges!


  lemons!

When the #foreach completes, the loop variable ($product in the example) goes back to whatever value it had before, or to being undefined if it was undefined before.

Within the #foreach, a special variable $foreach is defined, such that you can write $foreach.hasNext, which will be true if there are more values after this one or false if this is the last value. For example:

#foreach ($product in $allProducts)${product}#if ($foreach.hasNext), #end#end

This would produce the output oranges, lemons for the list above. (The example is scrunched up to avoid introducing extraneous spaces, as described in the section on spaces below.)

Velocity gives the $foreach variable other properties (index and count) but EscapeVelocity does not.

Macros

A macro is a part of the template that can be reused in more than one place, potentially with different parameters each time. In the simplest case, a macro has no arguments:

#macro (hello) bonjour #end

Then the macro can be referenced by writing #hello() and the result will be the string bonjour inserted at that point.

Macros can also have parameters:

#macro (greet $hello $world) $hello, $world! #end

Then #greet("bonjour", "monde") would produce bonjour, monde!. The comma is optional, so you could also write #greet("bonjour" "monde").

When a macro completes, the parameters ($hello and $world in the example) go back to whatever values they had before, or to being undefined if they were undefined before.

All macro definitions take effect before the template is evaluated, so you can use a macro at a point in the template that is before the point where it is defined. This also means that you can't define a macro conditionally:

## This doesn't work!
#if ($language == "French")
#macro (hello) bonjour #end
#else
#macro (hello) hello #end
#end

There is no particular reason to define the same macro more than once, but if you do it is the first definition that is retained. In the #if example just above, the bonjour version will always be used.

Macros can make templates hard to understand. You may prefer to put the logic in a Java method rather than a macro, and call the method from the template using $methods.doSomething("foo") or whatever.

Block quoting

If you have text that should be treated verbatim, you can enclose it in #[[...]]#. The text represented by ... will be copied into the output. # and $ characters will have no effect in that text.

#[[ This is not a #directive, and this is not a $variable. ]]#

Including other templates

If you want to include a template from another file, you can use the #parse directive. This can be useful if you have macros that are shared between templates, for example.

#set ($foo = "bar")
#parse("macros.vm")
#mymacro($foo) ## #mymacro defined in macros.vm

For this to work, you will need to tell EscapeVelocity how to find "resources" such as macro.vm in the example. You might use something like this:

ResourceOpener resourceOpener = resourceName -> {
  InputStream inputStream = getClass().getResource(resourceName);
  if (inputStream == null) {
    throw new IOException("Unknown resource: " + resourceName);
  }
  return new BufferedReader(InputStreamReader(inputStream, StandardCharsets.UTF_8));
};
Template template = Template.parseFrom("foo.vm", resourceOpener);

In this case, the resourceOpener is used to find the main template foo.vm, as well as any templates it may reference in #parse directives.

Spaces

For the most part, spaces and newlines in the template are preserved exactly in the output. To avoid unwanted newlines, you may end up using ## comments. In the #foreach example above we had this:

#foreach ($product in $allProducts)${product}#if ($foreach.hasNext), #end#end

That was to avoid introducing unwanted spaces and newlines. A more readable way to achieve the same result is this:

#foreach ($product in $allProducts)##
${product}##
#if ($foreach.hasNext), #end##
#end

Spaces are ignored between the # of a directive and the ) that closes it, so there is no trace in the output of the spaces in #foreach ($product in $allProducts) or #if ($foreach.hasNext). Spaces are also ignored inside references, such as $indexme[ $i ] or $callme( $i , $j ).

If you are concerned about the detailed formatting of the text from the template, you may want to post-process it. For example, if it is Java code, you could use a formatter such as google-java-format. Then you shouldn't have to worry about extraneous spaces.