java构建时执行代码,有点像C++11中constexpr
java构建时执行代码,有点像C++11中constexpr
Java 其它杂项
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详细介绍
@ConstExpr
Simulates constexpr
from C++11, build-time code-execution, from cppreference.com:
The constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time.
But Why?
- embed build-time variables directly into class files
- naive string obfuscation
- maven's resource filtering can be clunky for certain use-cases
Usage
The API is restricted to the @ConstExpr
annotation. The maven plugin does its thing using ASM.
On Fields
Fields must satisfy the following prerequisiites to be eligible:
static final
modifiers- primitive value or string.
The final value is written to the transformed class.
On Methods
In contrast to C++1x, @ConstExpr
on static methods turn them into build-time only methods. Consequently, do not annotate any methods required during runtime.
Sample
public class Exhibit {
@ConstExpr // recording the time at build
public static final long timestamp = System.currentTimeMillis();
@ConstExpr
public static final long seed = generateSeed();
@ConstExpr // this method will be completely removed
private static int generateSeed() {
String s = "hellooooo";
int sum = 0;
for (char c : s.toCharArray())
sum += c;
return new Random(sum).nextInt();
}
}
Usage: Maven
<dependencies>
<dependency>
<groupId>net.onedaybeard.constexpr</groupId>
<artifactId>constexpr-api</artifactId>
<version>0.1.0</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>net.onedaybeard.constexpr</groupId>
<artifactId>constexpr-maven-plugin</artifactId>
<version>0.1.0</version>
<executions>
<execution>
<id>transform</id>
<goals>
<goal>constexpr</goal>
</goals>
</execution>
</executions>
<!-- default configuration -->
<configuration>
<!-- property: constexpr.skip -->
<skip>false</skip>
<!-- property: constexpr.verbose -->
<verbose>false</verbose>
</configuration>
</plugin>
</plugins>
</build>
Running the plugin should result in output similar to:
[INFO] --- constexpr-maven-plugin:0.1.0:constexpr (transform) @ map ---
[INFO] Scanned 499 classes ............................................... 86ms
[INFO] Transformed 1 classes ............................................. 46ms
[INFO]
[INFO] @ConstExpr Log
[INFO] ------------------------------------------------------------------------
[INFO] s.f.m.u.BuildProperties ....................................... fields:1
[INFO] ------------------------------------------------------------------------
[INFO]
The Details
- scan class files for
@ConstExpr
and validate - for each annotated type:
- remove annotation
- resolve the value of each
@ConstExpr
field via reflection - write the result to the field signature
- remove old field initialization bytecode from the static initializer
- remove any methods annotated with
@ConstExpr
- if static initializer is left empty, remove it completely
- write transformed
byte[]
to source class file- (entries no longer required by the constant pool are cleared)
Bytecode: before and after
- Bytecode disassembly of PlainString.java, diff view.