RestTemplate底层使用的是java.net.URLConnection底层包，如果想其它诸如OkHttp,HttpClient，可以在项目里面配置。

RestTemplate继承体系图：

 

HttpAccessor 这是一个抽象类：也就是在这里默认使用SimpleClientHttpRequestFactory创建的request请求对象

RestOperations 这是一个接口，在这里则定义了项目中使用的getForObject，postForObject等方法，按照设计模式的单一职责原则来说，这里单独负责请求，而在RestTemplate对这些方法做出了实现


RestOperations接口代码：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
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package org.springframework.web.client;

import java.net.URI;
import java.util.Map;
import java.util.Set;
import org.springframework.core.ParameterizedTypeReference;
import org.springframework.http.HttpEntity;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpMethod;
import org.springframework.http.RequestEntity;
import org.springframework.http.ResponseEntity;
import org.springframework.lang.Nullable;

public interface RestOperations {
    @Nullable
    <T> T getForObject(String var1, Class<T> var2, Object... var3) throws RestClientException;

    @Nullable
    <T> T getForObject(String var1, Class<T> var2, Map<String, ?> var3) throws RestClientException;

    @Nullable
    <T> T getForObject(URI var1, Class<T> var2) throws RestClientException;

    <T> ResponseEntity<T> getForEntity(String var1, Class<T> var2, Object... var3) throws RestClientException;

    <T> ResponseEntity<T> getForEntity(String var1, Class<T> var2, Map<String, ?> var3) throws RestClientException;

    <T> ResponseEntity<T> getForEntity(URI var1, Class<T> var2) throws RestClientException;

    HttpHeaders headForHeaders(String var1, Object... var2) throws RestClientException;

    HttpHeaders headForHeaders(String var1, Map<String, ?> var2) throws RestClientException;

    HttpHeaders headForHeaders(URI var1) throws RestClientException;

    @Nullable
    URI postForLocation(String var1, @Nullable Object var2, Object... var3) throws RestClientException;

    @Nullable
    URI postForLocation(String var1, @Nullable Object var2, Map<String, ?> var3) throws RestClientException;

    @Nullable
    URI postForLocation(URI var1, @Nullable Object var2) throws RestClientException;

    @Nullable
    <T> T postForObject(String var1, @Nullable Object var2, Class<T> var3, Object... var4) throws RestClientException;

    @Nullable
    <T> T postForObject(String var1, @Nullable Object var2, Class<T> var3, Map<String, ?> var4) throws RestClientException;

    @Nullable
    <T> T postForObject(URI var1, @Nullable Object var2, Class<T> var3) throws RestClientException;

    <T> ResponseEntity<T> postForEntity(String var1, @Nullable Object var2, Class<T> var3, Object... var4) throws RestClientException;

    <T> ResponseEntity<T> postForEntity(String var1, @Nullable Object var2, Class<T> var3, Map<String, ?> var4) throws RestClientException;

    <T> ResponseEntity<T> postForEntity(URI var1, @Nullable Object var2, Class<T> var3) throws RestClientException;

    void put(String var1, @Nullable Object var2, Object... var3) throws RestClientException;

    void put(String var1, @Nullable Object var2, Map<String, ?> var3) throws RestClientException;

    void put(URI var1, @Nullable Object var2) throws RestClientException;

    @Nullable
    <T> T patchForObject(String var1, @Nullable Object var2, Class<T> var3, Object... var4) throws RestClientException;

    @Nullable
    <T> T patchForObject(String var1, @Nullable Object var2, Class<T> var3, Map<String, ?> var4) throws RestClientException;

    @Nullable
    <T> T patchForObject(URI var1, @Nullable Object var2, Class<T> var3) throws RestClientException;

    void delete(String var1, Object... var2) throws RestClientException;

    void delete(String var1, Map<String, ?> var2) throws RestClientException;

    void delete(URI var1) throws RestClientException;

    Set<HttpMethod> optionsForAllow(String var1, Object... var2) throws RestClientException;

    Set<HttpMethod> optionsForAllow(String var1, Map<String, ?> var2) throws RestClientException;

    Set<HttpMethod> optionsForAllow(URI var1) throws RestClientException;

    <T> ResponseEntity<T> exchange(String var1, HttpMethod var2, @Nullable HttpEntity<?> var3, Class<T> var4, Object... var5) throws RestClientException;

    <T> ResponseEntity<T> exchange(String var1, HttpMethod var2, @Nullable HttpEntity<?> var3, Class<T> var4, Map<String, ?> var5) throws RestClientException;

    <T> ResponseEntity<T> exchange(URI var1, HttpMethod var2, @Nullable HttpEntity<?> var3, Class<T> var4) throws RestClientException;

    <T> ResponseEntity<T> exchange(String var1, HttpMethod var2, @Nullable HttpEntity<?> var3, ParameterizedTypeReference<T> var4, Object... var5) throws RestClientException;

    <T> ResponseEntity<T> exchange(String var1, HttpMethod var2, @Nullable HttpEntity<?> var3, ParameterizedTypeReference<T> var4, Map<String, ?> var5) throws RestClientException;

    <T> ResponseEntity<T> exchange(URI var1, HttpMethod var2, @Nullable HttpEntity<?> var3, ParameterizedTypeReference<T> var4) throws RestClientException;

    <T> ResponseEntity<T> exchange(RequestEntity<?> var1, Class<T> var2) throws RestClientException;

    <T> ResponseEntity<T> exchange(RequestEntity<?> var1, ParameterizedTypeReference<T> var2) throws RestClientException;

    @Nullable
    <T> T execute(String var1, HttpMethod var2, @Nullable RequestCallback var3, @Nullable ResponseExtractor<T> var4, Object... var5) throws RestClientException;

    @Nullable
    <T> T execute(String var1, HttpMethod var2, @Nullable RequestCallback var3, @Nullable ResponseExtractor<T> var4, Map<String, ?> var5) throws RestClientException;

    @Nullable
    <T> T execute(URI var1, HttpMethod var2, @Nullable RequestCallback var3, @Nullable ResponseExtractor<T> var4) throws RestClientException;
}

通过这个入口查看 底层是通过通过类之间的关系来实现请求的创建

    @Nullable
    protected <T> T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback, @Nullable ResponseExtractor<T> responseExtractor) throws RestClientException {
        Assert.notNull(url, "URI is required");
        Assert.notNull(method, "HttpMethod is required");
        ClientHttpResponse response = null;

        Object var14;
        try {
            ClientHttpRequest request = this.createRequest(url, method);
            if (requestCallback != null) {
                requestCallback.doWithRequest(request);
            }

            response = request.execute();
            this.handleResponse(url, method, response);
            var14 = responseExtractor != null ? responseExtractor.extractData(response) : null;
        } catch (IOException var12) {
            String resource = url.toString();
            String query = url.getRawQuery();
            resource = query != null ? resource.substring(0, resource.indexOf(63)) : resource;
            throw new ResourceAccessException("I/O error on " + method.name() + " request for \"" + resource + "\": " + var12.getMessage(), var12);
        } finally {
            if (response != null) {
                response.close();
            }

        }

        return var14;
    }

 

 

客户端请求创建工厂的顶层接口类，这里是一个规范，需要诸如netty、okHttp做实现
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
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package org.springframework.http.client;

import java.io.IOException;
import java.net.URI;
import org.springframework.http.HttpMethod;

@FunctionalInterface
public interface ClientHttpRequestFactory {
    ClientHttpRequest createRequest(URI var1, HttpMethod var2) throws IOException;
}

 ClientHttpRequestFactory工厂的实现类：

本文我们这里只是关心RestTemplate默认使用的请求创建工厂SimpleClientHttpRequestFactory

    public ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException {
        HttpURLConnection connection = this.openConnection(uri.toURL(), this.proxy);
        this.prepareConnection(connection, httpMethod.name());
        return (ClientHttpRequest)(this.bufferRequestBody ? new SimpleBufferingClientHttpRequest(connection, this.outputStreaming) : new SimpleStreamingClientHttpRequest(connection, this.chunkSize, this.outputStreaming));
    }

通过跟踪

HttpURLConnection connection = this.openConnection(uri.toURL(), this.proxy);

URLConnection urlConnection = proxy != null ? url.openConnection(proxy) : url.openConnection();

通过三目表达式判断当前是否设置代理，但是不论是 url.openConnection(proxy) 还是url.openConnection()都是java.net.URL里面的方法，java.net.URL这个类经过final的修饰，已经不能被继承，

作为ClientHttpRequestFactory一个实现，我们通过查看其代码

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

package org.springframework.http.client;

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.Proxy;
import java.net.URI;
import java.net.URL;
import java.net.URLConnection;
import org.springframework.core.task.AsyncListenableTaskExecutor;
import org.springframework.http.HttpMethod;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;

public class SimpleClientHttpRequestFactory implements ClientHttpRequestFactory, AsyncClientHttpRequestFactory {
    private static final int DEFAULT_CHUNK_SIZE = 4096;
    @Nullable
    private Proxy proxy;
    private boolean bufferRequestBody = true;
    private int chunkSize = 4096;
    private int connectTimeout = -1;
    private int readTimeout = -1;
    private boolean outputStreaming = true;
    @Nullable
    private AsyncListenableTaskExecutor taskExecutor;

    public SimpleClientHttpRequestFactory() {
    }

    public void setProxy(Proxy proxy) {
        this.proxy = proxy;
    }

    public void setBufferRequestBody(boolean bufferRequestBody) {
        this.bufferRequestBody = bufferRequestBody;
    }

    public void setChunkSize(int chunkSize) {
        this.chunkSize = chunkSize;
    }

    public void setConnectTimeout(int connectTimeout) {
        this.connectTimeout = connectTimeout;
    }

    public void setReadTimeout(int readTimeout) {
        this.readTimeout = readTimeout;
    }

    public void setOutputStreaming(boolean outputStreaming) {
        this.outputStreaming = outputStreaming;
    }

    public void setTaskExecutor(AsyncListenableTaskExecutor taskExecutor) {
        this.taskExecutor = taskExecutor;
    }

    public ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException {
        HttpURLConnection connection = this.openConnection(uri.toURL(), this.proxy);
        this.prepareConnection(connection, httpMethod.name());
        return (ClientHttpRequest)(this.bufferRequestBody ? new SimpleBufferingClientHttpRequest(connection, this.outputStreaming) : new SimpleStreamingClientHttpRequest(connection, this.chunkSize, this.outputStreaming));
    }

    public AsyncClientHttpRequest createAsyncRequest(URI uri, HttpMethod httpMethod) throws IOException {
        Assert.state(this.taskExecutor != null, "Asynchronous execution requires TaskExecutor to be set");
        HttpURLConnection connection = this.openConnection(uri.toURL(), this.proxy);
        this.prepareConnection(connection, httpMethod.name());
        return (AsyncClientHttpRequest)(this.bufferRequestBody ? new SimpleBufferingAsyncClientHttpRequest(connection, this.outputStreaming, this.taskExecutor) : new SimpleStreamingAsyncClientHttpRequest(connection, this.chunkSize, this.outputStreaming, this.taskExecutor));
    }

    protected HttpURLConnection openConnection(URL url, @Nullable Proxy proxy) throws IOException {
        URLConnection urlConnection = proxy != null ? url.openConnection(proxy) : url.openConnection();
        if (!HttpURLConnection.class.isInstance(urlConnection)) {
            throw new IllegalStateException("HttpURLConnection required for [" + url + "] but got: " + urlConnection);
        } else {
            return (HttpURLConnection)urlConnection;
        }
    }

    protected void prepareConnection(HttpURLConnection connection, String httpMethod) throws IOException {
        if (this.connectTimeout >= 0) {
            connection.setConnectTimeout(this.connectTimeout);
        }

        if (this.readTimeout >= 0) {
            connection.setReadTimeout(this.readTimeout);
        }

        connection.setDoInput(true);
        if ("GET".equals(httpMethod)) {
            connection.setInstanceFollowRedirects(true);
        } else {
            connection.setInstanceFollowRedirects(false);
        }

        if (!"POST".equals(httpMethod) && !"PUT".equals(httpMethod) && !"PATCH".equals(httpMethod) && !"DELETE".equals(httpMethod)) {
            connection.setDoOutput(false);
        } else {
            connection.setDoOutput(true);
        }

        connection.setRequestMethod(httpMethod);
    }
}

 本文需要讲的内容就结束了，可以知道RestTemplate默认实现是使用java.net包里面的实现。这里在项目里面均可经过配置修改。