接上节继续，Checkpoint（检查点）的主要作用是保存图执行过程中的状态，让图可以在需要时暂停并在之后从断点恢复执行，通常需要与interrupt结合使用。

public static StateGraph<MessagesState<String>> getGraph() throws GraphStateException {
    return new StateGraph<>(MessagesState.SCHEMA, MessagesState<String>::new)
            .addNode("node-1", node_async(state -> Map.of(MESSAGES_STATE, "have")))
            .addNode("node-2", node_async(state -> Map.of(MESSAGES_STATE, "a")))
            .addNode("node-3", node_async(state -> Map.of(MESSAGES_STATE, "good")))
            .addNode("node-4", node_async(state -> Map.of(MESSAGES_STATE, "trip")))
            .addEdge(GraphDefinition.START, "node-1")
            .addEdge("node-1", "node-2")
            .addEdge("node-2", "node-3")
            .addEdge("node-3", "node-4")
            .addEdge("node-4", GraphDefinition.END);
}

static void startWithoutCheckpoint() throws Exception {
    StateGraph<MessagesState<String>> graph = getGraph();

    graph.addBeforeCallNodeHook((String node, MessagesState<String> data, RunnableConfig config) -> {
        out.println("Before calling node: " + node + ", data: " + data.data());
        return CompletableFuture.completedFuture(data.data());
    });

    //node-3进入前，被打断
    CompileConfig cc = CompileConfig.builder()
            .interruptBefore("node-3")
            .build();

    RunnableConfig rc = RunnableConfig.builder()
            .threadId("test-interrupt")
            .build();

    CompiledGraph<MessagesState<String>> workflow = graph.compile(cc);

    //运行完后，最终只会输出[have a] - node-3被打断，执行中止
    workflow.invoke(Map.of(), rc)
            .ifPresent(state -> System.out.println(state.value(MESSAGES_STATE).orElse(null)));
}

Before calling node: node-1, data: {messages=[]}
Before calling node: node-2, data: {messages=[have]}
[have, a]

可见，interrupt会让图提前中止。

仅仅设置interrupt断点通常没有太大的实际意义 ，可以结合CheckpointSaver在断点时，将图的状态保存下来，便于后续恢复。

static BaseCheckpointSaver getSaver() {
        return new MemorySaver();
//        return new FileSystemSaver(Path.of("output"), new ObjectStreamStateSerializer<>(MessagesState<String>::new));
//        return new JsonFileSystemSaver(Path.of("output"));
    }
    
    static void startWithCheckpoint(BaseCheckpointSaver saver) throws Exception {
        StateGraph<MessagesState<String>> graph = getGraph();

        graph.addBeforeCallNodeHook((String node, MessagesState<String> data, RunnableConfig config) -> {
            out.println("Before calling node: " + node + ", data: " + data.data());
            return CompletableFuture.completedFuture(data.data());
        });

        //node-3进入前，被打断
        CompileConfig cc = CompileConfig.builder()
                .checkpointSaver(saver)
                .interruptBefore("node-3")
                .build();

        RunnableConfig rc = RunnableConfig.builder().threadId("test-interrupt")
                .build();
        CompiledGraph<MessagesState<String>> workflow = graph.compile(cc);

        //运行完后，最终只会输出[have a] - node-3被打断，执行中止
        workflow.invoke(Map.of(), rc)
                .ifPresent(state -> System.out.println(state.value(MESSAGES_STATE).orElse(null)));
    }

    static void recoverFromCheckpoint(BaseCheckpointSaver saver) throws Exception {
        StateGraph<MessagesState<String>> graph = getGraph();
        graph.addBeforeCallNodeHook((String node, MessagesState<String> data, RunnableConfig config) -> {
            out.println("Before calling node: " + node + ", data: " + data.data());
            return CompletableFuture.completedFuture(data.data());
        });

        CompileConfig cc = CompileConfig.builder()
                .checkpointSaver(saver)
                .interruptBefore("node-3")
                .build();

        RunnableConfig rc = RunnableConfig.builder().threadId("test-interrupt")
                .build();
        CompiledGraph<MessagesState<String>> workflow = graph.compile(cc);

        //取出interrupt前的状态快照
        StateSnapshot<MessagesState<String>> snapshot = workflow.getState(rc);
        System.out.println("snapshot=>" + snapshot.state().data());

        //将图的状态，更新到interrupt前的状态快照
        RunnableConfig runnableConfig = workflow.updateState(rc, snapshot.state().data());

        //从断点恢复运行
        workflow.invoke(GraphInput.resume(), runnableConfig)
                .ifPresent(state -> System.out.println(state.value(MESSAGES_STATE).orElse(null)));
    }

接下来的recoverFromCheckpoint方法中，行47将先前保存的状态取出来，然后更新到图中（相当于恢复打断前的现场)，最后54行继续从断点处运行。

串在一起测试下：

public static void main(String[] args) throws Exception {
    startWithoutCheckpoint();
    out.println("\n------------------------\n");

    BaseCheckpointSaver saver = getSaver();
    startWithCheckpoint(saver);
    out.println("\n------------------------\n");

    recoverFromCheckpoint(saver);
}

Before calling node: node-1, data: {messages=[]}
Before calling node: node-2, data: {messages=[have]}
[have, a]

------------------------

Before calling node: node-1, data: {messages=[]}
Before calling node: node-2, data: {messages=[have]}
[have, a]

------------------------

snapshot=>{messages=[have, a]}
Before calling node: node-3, data: {messages=[have, a]}
Before calling node: node-4, data: {messages=[have, a, good]}
[have, a, good, trip]