java reflection.

发表于 2020-12-21

Method.invoke

调用链

method.invoke()
    ->
        MethodAccessor.invoke()
            -> ReflectionFactory.newMethodAccessor() -> DelegatingMethodAccessorImpl
            -> invoke
            -> DelegatingMethodAccessorImpl.invoke
                -- delegate = NativeMethodAccessorImpl
                                    -> invoke
                                        if(numInvocations > ReflectionFactory.inflationThreshold())
                                            -> acc = MethodAccessorGenerator.generateMethod
                                            -> parent.setDelegate(acc) 
                                        invoke0()//native invoke
                -- deletegate = sun.reflect.GeneratedMethodAccessor1@101df177

补充说明

下文中代理类皆为MethodAccessor的子类
当ReflectionFactory.noInflation = true 会直接生成代理类
反射调用方法执行次数大于ReflectionFactory.inflationThreshold(默认15)时，会生成代理类，用于提高反射执行效率,代理类反编译后源码大致如下(以String#toCharArray为例)

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package sun.reflect;

import java.lang.reflect.InvocationTargetException;

public class GeneratedMethodAccessor1 extends MethodAccessorImpl {
    public GeneratedMethodAccessor1() {
    }

    //var1 为原object对象，var为方法参数数组
    public Object invoke(Object var1, Object[] var2) throws InvocationTargetException {
        if (var1 == null) {
            throw new NullPointerException();
        } else {
            //生成字节码时已经自动处理了对象类型
            String var10000;
            try {
                var10000 = (String)var1;
                if (var2 != null && var2.length != 0) {
                    throw new IllegalArgumentException();
                }
            } catch (NullPointerException | ClassCastException var4) {
                throw new IllegalArgumentException(var4.toString());
            }

            try {
                //实际调用 等价于 strObj.toCharArray();所以开销很小
                return var10000.toCharArray();
            } catch (Throwable var3) {
                throw new InvocationTargetException(var3);
            }
        }
    }
}

当系统中反射调用过多时会导致metaspace溢出，因为生成了过多的代理类
noInflation可通过sun.reflect.noInflation配置
inflationThreshold可通过sun.reflect.inflationThreshold配置

Constructor.newInstance

调用链

和上文大致类似

补充说明

生成代理类的代理类反编译后源码大致如下(以new String(String)为例)

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package sun.reflect;

import java.lang.reflect.InvocationTargetException;

public class GeneratedConstructorAccessor2 extends ConstructorAccessorImpl {
    public GeneratedConstructorAccessor2() {
    }

    public Object newInstance(Object[] var1) throws InvocationTargetException {
        String var10000;
        String var10001;
        String var10002;
        try {
            var10000 = new String;
            var10001 = var10000;
            if (var1.length != 1) {
                throw new IllegalArgumentException();
            }

            var10002 = (String)var1[0];
        } catch (NullPointerException | ClassCastException var3) {
            throw new IllegalArgumentException(var3.toString());
        }

        try {
            var10001.<init>(var10002);
            return var10000;
        } catch (Throwable var2) {
            throw new InvocationTargetException(var2);
        }
    }
}

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