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Java反射API中的反射方法是什么？

发布于 2024-12-16 20:08:44 字数 53 浏览 2 评论 0原文

除了Java反射API之外，我们还可以使用什么方法呢？我们如何定义它？源代码示例会很有帮助。

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霓裳挽歌倾城醉 2024-12-23 20:08:44

JDK 1.6 具有动态编译 Java 类的能力（请参阅 getSystemJavaCompiler ）。如果您不想使用反射（出于性能原因，我们使用动态编译），那么这是一个合理的方法。

从包含代码的字符串创建 Java 源文件：

   public class JavaSourceFromString extends SimpleJavaFileObject {
       final String code;

       JavaSourceFromString(String name, String code) {
           super(URI.create("string:///" + name.replace('.','/') + Kind.SOURCE.extension),
                 Kind.SOURCE);
           this.code = code;
       }

       @Override
       public CharSequence getCharContent(boolean ignoreEncodingErrors) {
           return code;
       }
   }

static final String sourceCode = ""
        + "import org.example.MySomethingObject;"
        + "public class GetSomethingDynamically implements DynamicStringGetter {\n" // DynamicStringGetter would define getString as a standard way to get a String from an object
        + "    public String getString(Object o) {\n"
        + "        MySomethingObject obj = (MySomethingObject) o;\n"
        + "        return o.getSomething();\n"
        + "    }\n"
        + "}\n";

   JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
   StandardJavaFileManager fileManager = compiler.getStandardFileManager(null, null, null);

   List<JavaFileObject> jFiles = new ArrayList<JavaFileObject>();
   jFiles.add(new JavaSourceFromString("org.example.DynamicClass", sourceCode));

   compiler.getTask(null, fileManager, null, null, null, jFiles).call();

然后动态加载新创建的类文件。

或者，使用字节码操作（例如 ASM）动态创建类。

这两种方法都比标准反射复杂得多，但与反射相比可以显着提高性能。

JDK 1.6 has the ability to dynamically compile Java classes (see getSystemJavaCompiler). If you don't want to use reflection (we're using dynamic compilation for performance reasons) then this is a reasonable approach.

Create a Java source file from a string containing the code:

   public class JavaSourceFromString extends SimpleJavaFileObject {
       final String code;

       JavaSourceFromString(String name, String code) {
           super(URI.create("string:///" + name.replace('.','/') + Kind.SOURCE.extension),
                 Kind.SOURCE);
           this.code = code;
       }

       @Override
       public CharSequence getCharContent(boolean ignoreEncodingErrors) {
           return code;
       }
   }

static final String sourceCode = ""
        + "import org.example.MySomethingObject;"
        + "public class GetSomethingDynamically implements DynamicStringGetter {\n" // DynamicStringGetter would define getString as a standard way to get a String from an object
        + "    public String getString(Object o) {\n"
        + "        MySomethingObject obj = (MySomethingObject) o;\n"
        + "        return o.getSomething();\n"
        + "    }\n"
        + "}\n";

   JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
   StandardJavaFileManager fileManager = compiler.getStandardFileManager(null, null, null);

   List<JavaFileObject> jFiles = new ArrayList<JavaFileObject>();
   jFiles.add(new JavaSourceFromString("org.example.DynamicClass", sourceCode));

   compiler.getTask(null, fileManager, null, null, null, jFiles).call();

Then you load the newly created class files dynamically.

Alternatively, use byte code manipulation (such as ASM) to create classes on the fly.

Both of these methods are a good deal more complex than standard reflection, but can have significant performance improvements over reflection.

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