java9系列(七)Variable Handles

時間 2019-11-07

標籤 java9 java 系列 variable handles 欄目 Java 简体版

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Variable Handles

Variable Handles的API主要是用來取代java.util.concurrent.atomic包以及sun.misc.Unsafe類的功能。一個variable handle是一個variable的類型引用，用來在一系列訪問模式下來讀寫variable。支持的variable包括實例變量，靜態成員，數據元素等。Variable Handles須要依賴jvm的加強及編譯器的協助，即須要依賴java語言規範及jvm規範的升級。java

實例

目標類

public static class Demo {
        public int count = 1;
        protected long sum = 100;
        private String name = "init";
        public int[] arrayData = new int[]{3,5,7};

        @Override
        public String toString() {
            return "Demo{" +
                    "name='" + name + '\'' +
                    ", count=" + count +
                    ", sum=" + sum +
                    ", data=" + Arrays.toString(arrayData) +
                    '}';
        }
    }

訪問public成員

@Test
    public void testSetPublicField() throws NoSuchFieldException, IllegalAccessException {
        Demo instance = new Demo();
        VarHandle countHandle = MethodHandles.lookup()
                .in(Demo.class)
                .findVarHandle(Demo.class, "count", int.class);
        countHandle.set(instance,99);
        System.out.println(instance.count);
    }

輸出api

訪問proteced成員

@Test
    public void testSetProtectedField() throws NoSuchFieldException, IllegalAccessException {
        Demo instance = new Demo();
        VarHandle countHandle = MethodHandles.lookup()
                .in(Demo.class)
                .findVarHandle(Demo.class, "sum", long.class);
        countHandle.set(instance,99999);
        System.out.println(instance);
    }

輸出數組

Demo{name='init', count=1, sum=99999, data=[3, 5, 7]}

訪問private成員

@Test
    public void testSetPrivateField() throws NoSuchFieldException, IllegalAccessException {
        Demo instance = new Demo();
        VarHandle countHandle = MethodHandles.privateLookupIn(Demo.class,MethodHandles.lookup())
                .findVarHandle(Demo.class, "name", String.class);
        countHandle.set(instance,"hello world");
        System.out.println(instance);
    }

輸出oracle

Demo{name='hello world', count=1, sum=100, data=[3, 5, 7]}

訪問數組類型

@Test
    public void testSetArray(){
        Demo instance = new Demo();
        VarHandle arrayVarHandle = MethodHandles.arrayElementVarHandle(int[].class);
        arrayVarHandle.compareAndSet(instance.arrayData,0,3,100);
        arrayVarHandle.compareAndSet(instance.arrayData,1,5,300);
        System.out.println(instance);
    }

輸出jvm

Demo{name='init', count=1, sum=100, data=[100, 300, 7]}

access modes

主要的訪問模式有以下幾種：ide

read access modes

such as reading a variable with volatile memory ordering effects;
主要有以下幾個方法：get, getVolatile, getAcquire, getOpaque.

with memory semantics of reading as if the variable was declared non-{@code volatile}. Commonly referred to as plain read access.

getVolatile

用於讀取volatile修飾的變量

getAcquire

ensures that subsequent loads and stores are not reordered before this access.

getOpaque

accessed in program order, but with no assurance of memory ordering effects with respect to other threads.

write access modes

such as updating a variable with release memory ordering effects;
主要有以下幾個方法：set, setVolatile, setRelease, setOpaque.

atomic update access modes

such as a compare-and-set on a variable with volatile memory order effects for both read and writing;
主要有以下幾個方法：compareAndSet, weakCompareAndSetPlain, weakCompareAndSet, weakCompareAndSetAcquire, weakCompareAndSetRelease, compareAndExchangeAcquire, compareAndExchange, compareAndExchangeRelease, getAndSet, getAndSetAcquire, getAndSetRelease.

numeric atomic update access modes

such as get-and-add with plain memory order effects for writing and acquire memory order effects for reading.
主要有以下幾個方法：getAndAdd, getAndAddAcquire, getAndAddRelease

bitwise atomic update access modes

such as get-and-bitwise-and with release memory order effects for writing and plain memory order effects for reading.
主要有以下幾個方法：getAndBitwiseOr, getAndBitwiseOrAcquire, getAndBitwiseOrRelease, getAndBitwiseAnd, getAndBitwiseAndAcquire, getAndBitwiseAndRelease, getAndBitwiseXor, getAndBitwiseXorAcquire, getAndBitwiseXorRelease.